A current resonant power source apparatus includes a series circuit (Lr, P, C2) connected between switch elements Q1 and Q2 and a capacitor C1, a full-wave rectifying and smoothing circuit (D1, D2, C3) for providing a DC voltage, a control circuit of Q1 and Q2, a voltage detector 11 of the DC voltage, a current detector of the primary winding P, a soft-start time constant setting unit, and an ON time controller. If a voltage generated to a time constant is smaller than a set voltage, the ON time controller sets an ON time for Q1 and Q2 according to the DC voltage and soft-start signal, and if it is equal to or greater than the set voltage, sets one of the first and second ON times according to the current from the current detector.