To provide a seal structure of a casing preventing defective seals.

A relay body is installed in a casing 6 which is constituted of a base 1 and a case 2 adhered to the base 1. On the surface of the base 1, a sealant application part 3 containing a joint faces of the base 1 and the case 2 and a circumferential part of a terminal 5, is formed. The sealant application part 3 is formed into a step lower than other parts of the base 1 surface, and on the sealant application part 3, project walls 4, which extend along the joint faces of the base 1 and the case 2 at regular intervals in the almost orthogonal direction for the joint faces, are planted. The sealant applied to this sealant application part 3 turns around into the adjacent project walls 4 by means of capillary phenomenon, then flows into the joint face of the base 1 and the case 2.