fitting portion 18 does not apply force to the pressed portion 33 this returns the grommet 12 to the state shown in fig7 b in which the lower portion of the grommet 12 is not expanded next the pcv valve 10 is removed from the coupling hole 11 with the male thread 23 still halfway mated with the female thread 32 a the pcv valve 10 is removed from the coupling hole 11 with the grommet 12 as shown in the state of fig7 a alternatively the male thread 23 may be completely disengaged from the female screw hole 32 in this state the pcv valve 10 is removed from the coupling hole 11 with the grommet 12 engaged with the coupling hole 11 the following advantages are obtained in the pcv valve coupling structure 100 of the first embodiment 1 the male thread 23 which is formed on the fitting portion 18 of the pcv valve 10 is meshed with the female thread 32 a of the grommet 12 thus the pcv valve 10 is engaged with the grommet 12 without requiring a highly accurate fitting allowance the formation of the female screw hole 32 in the grommet 12 decreases the number of machining steps in comparison to when forming a female screw in the coupling hole 11.