To mount an assembling member on the mating side in the mechanically simple way, stably and surely without using a tool.

Retaining annuluses 3 with through holes 30 of a U-shape are provided at upper and lower rising flange parts 32 in the surface of one member, then the surface of a mating member is provided with bored holes having square spaces 40 suited for the outer shape of the retaining annuluses 3, long sideways spaces 41, 42 that have width suited for wall thickness of the rising flange parts 32 and are continuously extend in the horizontal lateral direction from upper and lower positions of the square spaces 40, and tongue-shape projecting pieces 43 having the bases on the stopping side of the long sideways spaces 41, 42, then the retaining annuluses 3 are slid into the long sideways spaces 41, 42 from the square spaces 40 receiving the annuluses 3 into the mating member, and the projecting pieces 43 are fitted into the through holes 30, also the rising flange parts 32 are fitted into the long sideways spaces 41, 42, thus the one assembling member 1 is combined and fixed to the mating assembling member 2.