A front shutter (13') comprises arm portions (132, 133) formed at both ends of an upper side of a door member (131). While one shaft portion (136) is formed on one of the arm portions (132) through a positioning portion (134), the other shaft portion (137) is formed on the other of the arm portions (133). The shaft portions (136, 137') are inserted into holes formed on a cut and raised piece (111) and a side wall (112) respectively. The cut and raised piece (111) and the side wall (112) rotatably supports the shaft portions (136, 137'). A gap between the arm portion (133) and the side wall (112) is wider because of no positioning portion between the arm portion (133) and the side wall (112). The shutter spring (14') not only gives force to the front shutter (13') in a closing direction but also presses the front shutter toward the cut and raised piece (111) to prevent the shutter from moving along a pivot defined by the shaft portions (136, 137').