PURPOSE: To prevent intrusion of fishing string into an opening at the rear of casing, by providing a fishing string catching section for catching the fishing string under wound state at the propeller shaft side while facing against the inner face of said opening.

CONSTITUTION: A cutter 23 is held between a thrust washer 19 for suporting a propeller 13 to a propeller shaft 12 and the boss 15 of the propeller 13. The cutter 23 is comprised of an arm section 23B extending radially from the fixing section 23A, click section 23C bending forward from the leading edge of said arm section 23B and extending to the position facing against the inner face of the rear opening 11A of casing 11 and a stopper section 23D projecting toward the inner face of said opening 11A at the leading edge section of said click section 23C. A fishing string catching section 23E is formed with the outer face of click section 23C and the stopper section 23D. Consequently a string is cut by the cutter 23 or wound at the catching section 23E and prevented from intrusion.