PURPOSE: To enable an operation of a screw type freezer to be expanded to the same operating range as that of not using any economizer by a method wherein a branch passage opened to a motor chamber is disposed in a passage for feeding intermediate cooling refrigerant and this branch passage is provided with a bypassing valve opened under a differential pressure between a suction side pressure and a discharging side pressure.

CONSTITUTION: In a screw type freezer provided with an economizer cycle, a passage 8 for feeding intermediate refrigerant is provided with a branch passage 9 opened into a motor chamber 1c of a screw type compressor 1. A bypassing valve 10 opened when a differential pressure between a suction side pressure and a discharging side pressure of the compressor 1 is higher than a specified high pressure is disposed in the midway part of the branch passage 9. As the bypassing valve 10 is opened, a part of the intermediate cooling refrigerant is fed from the feeding passage 8 into the motor chamber 1c through the branch passage 9 and then the motor is cooled. Due to this fact, an operation can be expanded to the same operating range as that having no economizer cycle.