PURPOSE: To exchange filters easily keeping a front door closed by providing a filter receptacle into which a filter for filtering air taken in is inserted and a filter to be inserted into this filter receptacle, at the outside of an air intake.

CONSTITUTION: A filter receptacle 3 is fixed at the outside of an air intake 1A provided in the front door 1 of a switch board with a gap of length T. A filter 2 and a filter frame 4 are made into one united body by attaching the filter 2 to the filter frame 4 at its peripheral part, and the filter 2 is inserted from upside into the gap of length T between the front door 1 and the filter receptacle 3. As the dimension T of this gap is made that of the sum of the thicknesses of the filter 2 and filter frame 4, the filter 2 sticks closely to the front door 1. Air taken in passes through the air ventilating bole 3A of the filter receptacle 3, air ventilating hole 4A of the filter frame 4, filter 2, and air intake 1A of the front door, and flows into the switch board.