To provide an indoor ventilation system, and its method wherein an indoor ventilation fan can actively utilize environment-friendly electric power or economically inexpensive electric power.

The indoor ventilation system is provided with an indoor ventilator driven by direct current power, an electric storage device, an AC/DC converter receiving alternating current power from an external electric power supply system, converting it into the direct current power, and outputting it, and a change-over switch capable of changing between a first operation mode supplying output of the AC/DC converter to the indoor ventilator and the electric storage device, and a second operation mode supplying output of the electric storage device to the indoor ventilator.