To provide a system provided with a protection cap which can protect a connector part from dust or the like, which is not in danger of being lost and has a structure that is not in an obstacle to test, by forming a band part for the protection cap fitted to the connector part and inserting the band part into the hole part of a main body part so that it is not pulled out.

The connector part 2 for preventing the invasion of dust or the like by installing the protection cap 3 when it is not used is provided at the front of the main body part 1. The protection cap 3 has the structure where the band part 3b is inserted into the hole part 6 provided at the front of the main body part 1. The band part 3b slides along the side of the main body part 1 and it is drawn in. Thus, the protection cap 3 can be removed. Namely, a handle part and the band part 3b forming a pull-out stop part at an end part are formed in the protection cap 3. When the cap is attached/detached to/from the connector part 2 by taking the handle part, the band part 3b inserted into the hole part 6 of the main body part 1 slides along the side of the main body part 1 from the direction of the pull-out stop part.