To prevent an outboard motor from being stolen by restricting the rotation of a clamp screw.

A lock pipe 20 is installed between the confronting inner side faces of two clamp brackets 11L and 11R, and slits 14La and 14Lb are formed at the periphery of the clamp screw 14L. When a locking key 311 is turned, a lock piece 22 is put horizontal, and a movable plate 23L moves left to cause a pressing spring 27L and a return spring 28L to contract, while a movable piece 26L also moves left, and the forefront 25La of a lock bar 25L protrudes to the left to generate the locked condition. If at this time, the clamp screw 14L is going to be slackened even though the positions of the slits 14La and 14Lb in the rotating direction are not mating with the position of the lock bar 25L, the two engage each other after a half turn as maximum so as to restrict the rotation of the clamp screw 14L.