To facilitate cleaning by making a unit partition wall in a lowermost stage detachable.

A hook member 34 provided with a fixed hook piece 38, and two hook members 35 provided with elastic hook pieces 42, are fixed at prescribed spaces in a depth direction onto a bottom wall 10B of a body mounted with a unit partition wall 31B in a lowermost stage. A hook groove 44 and two hook holes 45 are opened to a bottom plate 32 of raised bottom shape of the unit partition wall 31B. In attaching operation, the unit partition wall 31B is lowered to fit the elastic hook pieces 42 into the hook holes 45 and then pulled frontward. Then the inner edge of each hook hole 45 elastically deforms the elastic hook piece 42 and gets thereunder, and the front side of the bottom plate 32 moves frontward while sliding on the fixed hook piece 38. When pulled into a normal position, the unit partition wall 31 is pressed to the bottom wall 10B by the restoring resilience of both elastic hook pieces 42. At the same time, the fixed hook piece 38 is fitted and fixed into the hook groove 44. Detachment is performed in reverse operation.