To provide a filter lock device whose manufacturing cost is reduced, whose assembling process is facilitated and whose safety with respect to an operator is sufficiently secured.

A dent 4 is formed at the back cover 2 of the back surface of a device main body and a grating-like louver 5 is integrally formed with the cover 2 at the inner side of the dent 4. A filter 3 is formed so as to be somewhat larger than the inner size of the dent 4 of the cover 2. Then, when the filter 3 is attached to the dent 4, it is bent and attached. Thus, the filter 3 is locked in the dent 4 by the elasticity of the filter 3 itself and friction between the inside surface of the dent 4 without using a device or a member for pressing the filter 3.