An electric power source system in which a momentary interruption of the electric power supply does not occur when the direction of voltage conversion is switched by a voltage conversion device that is capable of bidirectional voltage conversion. The electric power source system includes a bidirectional switching regulator that selectively switches between the voltage conversion in the step-up direction from a low-voltage system to a high-voltage system and the voltage conversion in the step-down direction from the high-voltage system to the low-voltage system, and a linear regulator, connected in parallel to the bidirectional switching regulator, that converts voltage in the step-down direction. The direction of current that flows via the bidirectional switching regulator switches from the step-up direction to the step-down direction after current flows in the step-down direction via the linear regulator.