In an auxiliary chamber gas supplying device for a gas engine including an auxiliary chamber gas supply path (5, 14, 28, and 29) through which fuel gas is supplied to an auxiliary chamber (4), the auxiliary chamber gas supply path is connected to a fuel gas supply source (24) through a solenoid valve (23), and when the solenoid valve is opened, the fuel gas, in an amount corresponding to an opened period of the solenoid valve, is supplied from the fuel gas supply source to the auxiliary chamber through the auxiliary chamber gas supply path, and the opened period of the solenoid valve is set in such a manner that a predetermined amount of fuel gas is supplied to the auxiliary chamber, and a restriction unit (30) that extends the opened period of the solenoid valve is formed in the auxiliary chamber gas supply path (5, 14, 28, and 29).