In a portable power source system comprising a battery pack for accommodating at least one secondary battery and a mounting part for mounting the battery pack, the mounting part is disposed in power using equipment, the battery pack comprises a charge circuit having a charge terminal and a discharge circuit having a discharge terminal, the charge circuit comprises a control circuit for controlling a voltage and a current during charging, the mounting part comprises a protruding external terminal for connecting with the discharge terminal, the battery pack comprises an inserting part for inserting the external terminal, the discharge terminal is disposed in a concealed position inside the inserting part, the battery pack is movable from an initial position to a fixing position while the external terminal has been inserted in the inserting part, and connection between the external terminal and the discharge terminal is achieved at the fixing position.