A plastic screw grommet includes a head portion provided with a screwing hole for a tapping screw, and a leg portion expanded or prevented from deflecting by screwing the tapping screw. A hole diameter of the screwing hole is made smaller than an outer diameter of the above-mentioned tapping screw and larger than a root diameter of the tapping screw. Also, a projecting portion for filling a screw groove of the tapping screw to be screwed is formed in an inner face of the screwing hole.