An electric apparatus comprising: a casing having its interior divided into a dust-proof region and an open region, said casing having a first opening formed in a portion of said dust-proof region; a cover plate; and a heat sink (32, 32a); wherein said cover plate is detachable with respect to said first opening; a ventilation hole is formed through said cover plate; and a ventilation path forming member (44, 44a) is mounted on an inner surface of said cover plate, said ventilation path forming member (44, 44a) forming a ventilation path in an interior thereof through which air flowing through said ventilation hole flows, said ventilation path forming member (44, 44a) including a heat-conductive cover rear wall (46, 46a), said cover rear wall being spaced from said inner surface of said cover plate so that the ventilation path is formed between said cover plate and said cover rear wall; wherein the ventilation hole is formed through said cover plate to communicate with said ventilation path; characterised in that: the cover plate covers said first opening; the heat sink (32, 32a) is disposed in said dust-proof region of said casing; the cooling-requiring components are mounted within said heat sink (32, 32a); said heat sink (32, 32a) has a second opening facing said first opening, and the heat-conductive cover rear wall (46, 46a) closes said second opening when said cover plate covers said first opening. a casing having its interior divided into a dust-proof region and an open region, said casing having a first opening formed in a portion of said dust-proof region; a cover plate; and a heat sink (32, 32a); wherein said cover plate is detachable with respect to said first opening; a ventilation hole is formed through said cover plate; and a ventilation path forming member (44, 44a) is mounted on an inner surface of said cover plate, said ventilation path forming member (44, 44a) forming a ventilation path in an interior thereof through which air flowing through said ventilation hole flows, said ventilation path forming member (44, 44a) including a heat-conductive cover rear wall (46, 46a), said cover rear wall being spaced from said inner surface of said cover plate so that the ventilation path is formed between said cover plate and said cover rear wall; wherein the ventilation hole is formed through said cover plate to communicate with said ventilation path; characterised in that: the cover plate covers said first opening; the heat sink (32, 32a) is disposed in said dust-proof region of said casing; the cooling-requiring components are mounted within said heat sink (32, 32a); said heat sink (32, 32a) has a second opening facing said first opening, and the heat-conductive cover rear wall (46, 46a) closes said second opening when said cover plate covers said first opening. the cover plate covers said first opening; the heat sink (32, 32a) is disposed in said dust-proof region of said casing; the cooling-requiring components are mounted within said heat sink (32, 32a); said heat sink (32, 32a) has a second opening facing said first opening, and the heat-conductive cover rear wall (46, 46a) closes said second opening when said cover plate covers said first opening.