To provide a spacer for screw fall prevention and a screw fall preventive structure capable of holding a screw with head so as not to fall on a main member even if the screw with head is loosened or coming off the screw hole of an auxiliary member after the main member is tightened to the auxiliary member with the screw with head.

The head part 61 of the screw 60 with head is stored, in a locked state, in a space S formed by the spacer main part 21 and the head part retaining part 23 of the spacer 20 opposed to each other. When the screw 60 with head inserted into the insertion hole 121 of the main member 110 is threaded into the screw hole 161 of the auxiliary member 150, the engagement claw 27 of the spacer 20 becomes engageable with the locked surface 125 of the main member 110, and the spacer 20 is held by the main member 110.