PURPOSE: To prevent a toner leak without using a filter liable to cause plugging by forming a recess facing an opening section on a cover member, and sticking a sheet material partitioning the opening section and recess to the cover member.

CONSTITUTION: A rectangular elastic member 19 made of sponge is stuck to the periphery section of the inner bottom face of the box-shaped cover section 18 of a cover member 17 opened on the end face. An opening section 19a is formed at the center section of the elastic member 19. The elastic member 19 reaches a portion equal to or lower than the upper edge section 18a of the cover section 18. A sheet material 20 is airtightly stuck to the upper edge section 18a of the cover section 18, and it is preferably made of an expandable material. A through hole 18b for communicating the opening section 19a and the inner space of an image forming device is bored on the bottom section of the cover section 18. A better effect can be obtained against the internal pressure rise due to the elastic deformation of the elastic member 19.