A connecting member comprises a rod-shaped body having a head and/or thread on the outer periphery for integrally connecting a plurality of members and a containing hole opened at one end thereof and extended longitudinally, a rod disposed in the hole so as to fix at its inner end to the inner end of the hole, a transmissive window fixed to the opening of the hole, and a fluid holding member holding a colored fluid and pressed by the open side end of the rod on the window. When a longitudinal strain developed in the body the rod release the holding member from under pressure so that colored fluid covers the open side end of the rod.