PURPOSE: To provide a waterproof grommet screw which always secure high sealability, and actually attains reduction of a cost.

CONSTITUTION: A screw is to be inserted and fixed to one installation hole 1a of a connection item 1, while one more connection item is fastened to the connection item 1. A grommet screw main body 10 has a packing 11 on an inner side of a flange 10a on an end portion, for abutting against the connection item 1. A temporary fitting first fin 12 is extendably arranged on the side part of the main body 10, and engaged with the connection item 1. A holding second fin 12 is arranged on the side part of the main body 10 in an extendable manner oppositely to the first fin 12. A plurality of claws 14 have tapered surfaces 14a formed in the vicinity of the flange 10a. A cut surface is formed on the upper side of each claw 14 with an angle of 20 degrees.