A gear device for electric motor 1 which is combined with an electric motor installed in a vehicle, wherein a breather pathway 8 is provided in a gear box 2 where a row of gears for transmitting force of the electric motor is contained, the breather pathway connecting between an inside and outside of the gear box 2, and in the breather pathway 8, a closing valve system 10 is provided to close the breather pathway 8 when an outer pressure of the gear box 2 is lower in comparison to an inner pressure of the gear box 2 exceeding an acceptable range.