PURPOSE: To improve the reliability of a relay in quick response to a user's request by forming a dial cover into a specific shape.

CONSTITUTION: An adjusting dial 4 is housed in a relay body case at a position deep from the upper surface thereof. A transparent dial cover 3 made of casting resin is formed into a thick disc having an engagement groove 3a on the external surface thereof. In addition, an L-shaped grip 3b is formed on the upper surface of the cover 3, and a through-hole for leading a metal wire material is formed on the forward end of the grip 3b. The cover 3, when concealing the dial 4 inserted in the adjusting dial insertion hole 4c of the case, is housed in the insertion hole 2a of a display cover 2 and then placed on an engagement projection 2c projected toward the inner side of the hole 2a. Then, a force is applied to the cover 3, and the projection 2c is pushed toward the dial 4, thereby causing the projection 2c to be engaged with the engagement groove 3a. When the cover 3 is formed according to the above construction, the insertion of a tool and the removal of the cover 3 are prevented by cutting out the grip 3b from the root thereof.