PURPOSE: In a screw propeller for vessel, to reduce exciting force by varying the screw disc pitch with single rotation period continuously in accordance to the flow speed distribution thereby maintaining the attack angle against flow constant.

CONSTITUTION: Upon rotation of screw propeller 1, each propeller disc 3 will swing while engaging the pitch control projections 7a, 7b at the tip with the pitch control grooves 6a, 6b in the inner wall of propeller frame 2. Upon swinging of disc 3 along the pitch control grooves 6a, 6b, the disc 3 having disc fixing section 4 fixed variably of pitch to the boss section 10 is controlled to low pitch at the position of high associated flow factor while to high pitch at low flow factor position and to control the pitch continuously with single rotary period of propeller such that the attack angle against water flow will be approximately constant. Consequently moment functioning onto the disc can be stabilized and the exciting force can be reduced considerably.