To display or post the replacement of a filter arranged in a flow path for cooling an internal heating element by forced ventilation.

A door 1 with a ventilation hole 13 with a filter 14 is equipped with a door window 1h and an unillustrated door hole 1b with a cover. The door 1 is provided with a display 2 consisting of an inflow hole 2s, mounted by a frame 2e, communicating with the door hole 1b and a transparent cylinder 2a storing a floater 2a between air current regulating plates 2c, 2d. A flow path for the display 2 is arranged in parallel with a flow path for the filter 14. If the ventilation resistance of the filter 14 is increased, the floater 2b ascends in the transparent cylinder 2a to indicate time for the replacement of the filter 14 as viewed through the door 1h.