PURPOSE: To prevent the rotational speed from varying caused by the external water or the like and improve detecting accuracy by covering the front and at least the upper front of a paddle in the flowing direction of fluid with a cover member.

CONSTITUTION: A speed detector 30 is constituted such that a cover member 33 is separately attached to a sensor body 30a having a paddle rotatably disposed on a support member 31. The cover member 33 made of polypropylene is constructed by a front wall 33a covering the front of the paddle 32, a top wall 33b covering above the paddle, side walls 33c, 33d covering the left and right sides of the support member 31 and a bottom wall 33d following the sidewall 33c and turned to the lower surface of a flange part 31a of the support member 31 which are all formed integrally. Since the front and upper of the paddle 32 are cover with the cover member 33, a splash of engine cooling water or the like drained to a jet pump chamber 29 is not splashed directly on the paddle 32. Thus, the rotational speed of the paddle 32 can be prevented from varying caused by the splash of the drain water or the like to improve the accuracy in detection.