A watertight cap used with an electronic apparatus is provided with a cap body to be set in an installing recess of the apparatus case of the electronic apparatus, an insertion part provided on the cap body and inserted into a through hole formed in the installing recess, a watertight projection provided on the peripheral surface of the insertion part to be resiliently pressed against the surface defining the through hole, a reinforce member embedded in the cap body and the insertion part for preventing the insertion part from deforming inwards, and a hook part provided on the insertion part so as to project into the apparatus case to engage with the stopping part in the through hole, when the insertion part is inserted in the through hole, thereby securing enhanced watertight of the apparatus case and preventing the cap body from slipping out from the apparatus case accidentally.