A power supply system of the present invention aims to achieve optimization of the efficiency and therefore includes: z (z is a natural number equal to or larger than 2) power supplies (PS-1 to PS-z) connected in parallel; and a controller (8) for the number of power supplies in operation which controls the number of power supplies in operation among the power supplies (PS-1 to PS-z). The controller (8) for the number of power supplies in operation determines the number of the power supplies in operation based on values of intersection currents i1 to i(z-1) which are determined through processes of: identifying output loss characteristics with respect to load currents of the z power supplies (PS-1 to PS-z) by use of convex functions f1 to fz, respectively; obtaining a function hn expressing a conversion efficiency of a total load current at the time of operating the n (n is a natural number equal to or smaller than z) power supplies (PS-1 to PS-z) based on the functions f1 to fz; and obtaining an intersection current iq (q is a natural number equal to or smaller than (z−1)) which is a current of an intersection point between a function hq and a function h(q+1). (Greek letter eta is replaced with h.)