To provide an oil injection type compressor capable of restricting the separation of water from the compressed gas without requiring a large space and increasing the energy consumption, and provide an operating method thereof.

This oil injection type compressor 1A is provided with an oil separating and recovering unit 13 in a discharge flow passage 12 of a compressor main body 11, and an oil flow passage 15 for leading the oil of an oil reservoir 14 in a lower part of the oil separating and recovering unit 13 to oil pouring positions such as a bearing and a shaft seal part and a gas compressing space inside of the compressor main body 11. This oil injection type compressor 1A is also provided with an opening and closing valve 17 for controlling the oil supply quantity to the oil pouring positions at a value smaller than the value of the normal operation and larger than the necessary minimum value till the preset time passes after starting the compressor is provided in a flow passage 15B as a branch part of the oil flow passage 15.