A connector-mounting structure includes a case and a connector mounted on the case. The connector has a connector housing, a terminal, a waterproof elastic member, and a connector holding plate fixed to an inner surface of the case. An outer size of the connector housing is larger than a hole size of a through-hole of the case. The waterproof elastic member has a sealing portion. A size of a recess of the connector holding plate is larger than an outer size of a flange portion of the connector housing. A hole size of a housing insertion hole of the connector holding plate is larger than an outer circumferential size of a main body of the connector housing. An outer circumferential size of the insertion portion is smaller than the hole size of the through-hole.