PURPOSE: To reduce a wearing of a seal member by a method wherein a projection is formed at a circumferential edge other than an upper surface of a circumferential wall of an auxiliary container, and a circumferential edge of a recess is provided with a seal member abutted against the projection when the circumferential wall of the auxiliary container occupies a positional relationship to close the recess.

CONSTITUTION: As an auxiliary container 14 is moved slightly in a rearward direction, a projection 15 is released from a resilient deformation part 17b of a seal packing 17, resulting in that the seal packing 17 shows a state in which it is spaced apart from a bottom surface, a left side surface and a right side surface of the auxiliary container 14. In this way, the projection 15 is arranged at a circumferential edge of a rear wall 14b of the auxiliary container 14. When the rear wall 14b and a rear wall 11a of a storage container 11 are coincided to each other, the resilient deformation part 17b arranged at a recess 16 is resiliently abutted against a projection 17 so as to prevent any clearance from being formed between the recess 15 and the auxiliary container 14. Accordingly, as the auxiliary container 14 is moved slightly in a rearward direction, the resilient deformed part 17b is spaced apart from a circumference of the auxiliary container 14, resulting in that it becomes possible to slide and move the auxiliary container 14 smoothly with a slight force. In addition, a wearing of the seal packing 17 is substantially reduced as compared with that obtained always under a sliding contact of the seal packing with the auxiliary container.