To provide a discharge-pressure variable screw supercharger capable of changing a discharge pressure in accordance with a demanded supercharging pressure, making performance in a high pressure region and performance in a low pressure region compatible with each other and free to stably work at a high temperature in a no-lubricating state.

A discharge port 10 is constituted of a low pressure port 10a of which the volume between rotors is relatively large and a high pressure port 10b of which the volume between rotors is relatively small. Furthermore, the supercharger is furnished with a closing valve 12 to open the low pressure port when an inside pressure is high owing to a pressure difference P between the inside of the low pressure port and the discharge port, and to close the low pressure port when the inside pressure is low.