PURPOSE: To prevent the contact failure of a relay group at one time even the relay group is composed of relays different in relay heights, in a preventive tool for relay contact failure preventing the occurrence of contact failure due to the relay falling off from a socket group.

CONSTITUTION: When relays are inserted into a socket group 3, mounted on an inner plate 2 in a relay unit 1 to be used, a fixed plate 5 is provided along the outer cover surface of relays 4 so as not to cause contact failure due to the falling off of a relay. Also an adjusting rod inserting hole 11 is drilled on the fixed plate 5 by conforming the hole 11 to fitting position of the socket 3, and when a shorter relay 4b is mounted, an adjusting rod 10 is fitted to the position.