A water injection type screw compressor wherein gas sucked from an intake flow is supplied to a rotor chamber, compressed and discharged into a discharge channel as compressed fluid together with water supplied to the rotor chamber, comprises a water separator disposed in the discharge channel to separate water and gas from compressed fluid, a water channel connecting the water separator to a compressor main body for supplying the rotor chamber with the water separated in the water separator, and an oil circulation channel including an oil pump, an oil filter, and a housing for storing oil for supplying the oil to where lubrication is needed. Further, a part of the water channel is arranged passing through an oil trap formed at the bottom inside the housing for storing oil. Thus, oil temperature increases can be minimized in simplified structure having no oil cooling means such as an oil cooler.