NOVELTY - The system has a bypass valve-control unit (106) controlling a by-pass valve based on stack supply flow rate and target stack supply flow rate by fixing a valve opening of the by-pass valve when the stack supply flow rate is settled in preset by-pass-valve control range set on basis of the target stack supply flow rate or limiting drive of the by-pass valve. A deregulation unit cancels control of the by-pass valve by the bypass valve-control unit when stack requirement compressor supply flow rate is larger than minimum flow volume at a predetermined value or more.    USE - Fuel cell system for a motor vehicle.    ADVANTAGE - The deregulation unit releases cancels control of the by-pass valve by the control unit when the stack requirement compressor supply flow rate is larger than the minimum flow volume at the predetermined value, so that real stack supply flow rate increases by closing the by-pass valve, thus enabling compressor control to reduce real stack supply flow rate toward target stack supply flow rate to reduce stack requirement compressor supply flow rate, and hence reducing compressor power consumption and fuel consumption. The bypass valve-control unit fixes the by-pass valve when the stack supply flow rate is settled in the predetermined by-pass-valve control range set on the basis of the target stack supply flow rate or limits drive of the by-pass valve, so that valve opening of the by-pass valve is fixed when real stack supply flow rate is settled in the predetermined by-pass-valve control range on the basis of target stack supply flow rate if real stack supply flow rate is larger than target stack supply flow rate, thus suppressing repetition of opening and closing of the by-pass valve at target stack supply flow rate vicinity.    DETAILED DESCRIPTION - The deregulation unit is a by-pass-valve unfixing signal output part (107).    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a cathode system of a fuel cell system.'(Drawing includes non-English language text)'    Target stack supply flow rate calculation part (102)    Target compressor supply-flow-rate setting part (104)    Cathode compressor control part (105)    Bypass valve-control unit (106)    By-pass-valve unfixing signal output part (107)