To provide a hybrid power source system in which either a solar cell or a fuel cell and a secondary cell are combined with each other, and the secondary cell is used as an electric power buffer and which can maintain a high energy efficiency even when there is a change in a charging state of the secondary cell, a change in an operating condition, or a secular change in a member, prevent overcharging of the secondary cell, and suppress thermal deterioration of the solar cell or the fuel cell due to generation of a surplus electric power unable to be taken out. A hybrid power source system is composed of a solar cell module 1 or a fuel cell module, a DC/DC converter (2) which supplies a suitable voltage to a load (3) and a secondary cell (4) after converting an electric power generated by the module into the suitable voltage, the secondary cell (4), and a shunt circuit (a constant voltage diode (6) or a shunt regulator IC) which is connected in parallel with the secondary cell (4), and which, when the secondary cell (4) is substantially in a full charging state, transforms substantially all of a surplus electric power which is not consumed in the load (3), of the generated electric power, into heat to abandon the resulting heat.