PURPOSE: To prevent generation of abnormal sound and abnormal vibration which occur by collision of fellow rotor gears at the time of inversion of the positive/negative of torque, by forming a screw rotor in this title, which has a non symmetrical gear profile which is a gear profile by which the total sum of intake torques of a female rotor formed on a non driving side is made negative.

CONSTITUTION: This rotor used for a screw type pump device consists of a pair of female/male rotor 1, 2 engaged each other. Negative torque acts always on a female rotor 1, and it is formed so that the forward side gear surface 1f of the female rotor gear 1a is brought in contact with the following side gear surface 2r of the male rotor gear 2a so as to rotate the female rotor l following the male rotor 2. Namely, each of rotor 1, 2 is formed into a non symmetrical gears, profile, that is a gear profile formed so that the total sum of intake torques of the female rotor 1 formed on a non driving side is made negative. Thereby, collision between fellow rotor gears which occur at the time of inversion of the positive/negative of torque is eliminated, so that it is possible to prevent generation of abnormal noise and abnormal vibration following the collision.