NOVELTY - The apparatus has a smooth capacitor connected between positive electrode side terminals and negative electrode side terminals. An inductor is connected with a pair of power semiconductor units of a primary side end between the positive electrode side terminals. A control unit (3) switches on a switch-on circuit whose resistance value differs when electric current of the inductor stays at a positive polarity or negative polarity at time of operation of a conversion main circuit (2) such that electric charge speed of gate capacitance of a switch element of the semiconductor unit increases.    USE - Direct current (DC)/DC voltage conversion apparatus for use in a power conversion system i.e. electric drive system, of a hybrid vehicle or electric vehicle for carrying out DC/DC conversion of voltage of a secondary battery e.g. lithium ion battery, nickel-hydrogen battery, and lead battery. Can also be used in a power conversion system for photovoltaic power generation and air-conditioner for carrying out DC/DC conversion of voltage of a solar cell and a fuel cell.    ADVANTAGE - The control unit switches on the switch-on circuit at the time of turn-on operation of an insulated gate bipolar transistor (IGBT) and outputs a gate drive signal based on magnitude correlation with a threshold value of inductor electric current, so that switching losses that arise in the IGBT and a PiN diode differ and switching loss of the power semiconductor unit becomes discontinuous when positive polarity and negative polarity range over zero during switching the switch-on circuit, thus suppressing transient voltage variation.    DETAILED DESCRIPTION - The switch element is FET. An INDEPENDENT CLAIM is also included for a voltage conversion control method.    DESCRIPTION OF DRAWING(S) - The drawing shows a circuit block diagram of a control unit and a conversion main circuit.'(Drawing includes non-English language text)'    Conversion main circuit (2)    Control unit (3)    Gate drive part (12)    Gate power supply circuit (13)    Gate drive circuits (121-124)