A chamber (2A) adapted to dismountably mount respective units (10) and (10) is formed in a main body (2). Guiding members (37, 38, 39) are formed on each of the opposite side walls of the chamber, and guided portions (11A, 11B, 24A, 24B) formed on opposite sides of the respective units (10, 20) are engaged with the guiding members so that they are guided to the mounting position. Near the initial portions (37A, 38A, 39A) of the respective guiding members (37, 38, 39) in the main body (2), an indicating member (41) including indicators (41a, 41b, 41c) which indicates initial positions of the respective guiding member (37, 38, 39). Consequently, the initial positions of the guided members (11A, 11B, 24A, 24B) of the respective units (10, 20) can be visually confirmed by the indicators (41a, 41b, 41c,) which improves the operability of insertion of the respective units (10, 20).