To prevent easy detachment of two members engaged with each other even though both the members are rather forcedly separated where an engagement hole formed on one of the members and an engagement protrusion part formed on the other member are engaged for connecting two members, by forming a protrusion contacting outer circumferential edge of the engagement hole and preventing both the members from separating by the contact of the protrusion to the outer circumferential edge of the engagement hole.

The inward protrusion 5b is formed on the engagement protrusion part 2 and placed at a level preventing contacting to inner circumferential edge of the engagement hole 4 when both the members 1, 3 are engaged with each other, and contacting to the inner circumferential edge of the engagement hole 4 when the engagement protrusion part 2 bends.