NOVELTY - A fuel cell system comprises a computing device configured to compute an amount of an oxidant gas required for first and second scavenging according to a system shut off time between a termination of power generation by a fuel cell and a start-up of a first scavenging device, where scavenging device conducts the first and second scavenging with the amount of the oxidant gas obtained by the computing device.    USE - The fuel cell system is for mounting on a vehicle, vessels, airplanes, and power generating systems of fixed use.    ADVANTAGE - The fuel cell system enables a reduction in power consumption.    DETAILED DESCRIPTION - A fuel cell system comprises:    (1) a fuel cell, which generates power with reaction between a fuel gas and an oxidant gas;    (2) a fuel gas passage through which the fuel gas flows;    (3) an oxidant gas passage through which the oxidant gas flows;    (4) a communicating passage configured to allow the fuel gas passage and the oxidant gas passage to communicate with each other;    (5) a communicating valve configured to open and close the communicating passage;    (6) a monitoring device configured to monitor a state transition of the fuel cell after a termination of power generation;    (7) a valve controller configured to open the communicating valve when a signal indicative of the state transition monitored by the monitoring device meets a predetermined criterion;    (8) a scavenging device configured to scavenge with the oxidant gas, and comprising a first scavenging device configured to conduct first scavenging for the oxidant gas passage, and a remaining hydrogen gas in a fuel electrode of the fuel cell; and a second scavenging device configured to conduct second scavenging for the fuel gas passage after the first scavenging device is started up, and scavenging for the remaining hydrogen gas in the fuel electrode with more amount than the first scavenging device; and    (9) a computing device configured to compute an amount of the oxidant gas required for the first and second scavenging according to a system shut off time between a termination of power generation by the fuel cell and a start-up of the first scavenging device, where scavenging device conducts the first and second scavenging with the amount of the oxidant gas obtained by the computing device.    An INDEPENDENT CLAIM is also included for scavenging a fuel cell system, which comprises a communicating passage and a communicating valve to selectively allow a fuel gas passage and an oxidant gas passage to communicate with each other, comprising:    (A) monitoring a state transition of the fuel cell after a termination of power generation;    (B) opening the communicating valve to open the communicating passage when a signal indicative of the state transition meets a predetermined criterion;    (C) computing an amount of an oxidant gas required for first and second scavenging according to a system shut off time between a termination of power generation by the fuel cell and a start-up of the first scavenging;    (D) conducting the first scavenging for the oxidant gas passage with the oxidant gas; and    (E) conducting the second scavenging for the fuel gas passage with the oxidant gas after a termination of the first scavenging, where first and second scavenging is conducted with the amount of the oxidant gas provided by the step of computing.    DESCRIPTION OF DRAWING(S) - The figure is a block diagram showing an overall architecture of a fuel cell system.