To provide a reel rotation speed detector for a slot game machine that can shorten a player's stand-by time.

The reel rotation speed detector for the slot game machine includes: a reel (R) rotationally driven by a stepping motor (11); light shielding plate pieces (19a)-(19d) for indicating the rotation original position of the reel; and a transmission type photo sensor (16) for detecting the passing of the light shielding plate pieces. The reel rotation speed detector detects the rotation speed of the reel by counting the number of steps of the motor (11) from the time when the photo sensor detects the passing of the light shielding plate pieces. A plurality of light shielding plate pieces are distributed interspersed on fixed P.C.D. (Pitch Circle Diameter) (D) of the reel and provided continuously and integrally projecting by a fixed length (L) into the reel from the intermediate parts of radial arms (9) in the reel. Only one of the light shielding plate pieces is assigned as the light shielding plate piece (19a) for detecting the rotation original position of the reel, and its width (W1) is set to be different from the width (W2) of the remaining light shielding plate pieces (19b)-(19d).