A control device of a power supply system includes a processor configured to set a sampling period, at which measurement values of a reactor current flowing through a reactor of a DC-DC converter and measured by an ammeter are sampled, so as to minimize a sum of differences between the length of a first period in a switching cycle of a switching element of the DC-DC converter and an integer multiple of the sampling period and between the length of a second period in the switching cycle and an integer multiple of the sampling period, the reactor current increasing during the first period and decreasing during the second period; sample, at intervals of the sampling period, measurement values of the reactor current measured by the ammeter; and average measurement values sampled in the switching cycle, thereby measuring an average of the reactor current in the switching cycle.