In a clean unit in which an active dust filter is used to keep a work chamber as a clean environment, the cleanliness of the work chamber depends upon 1/γ where γ is the dust collection efficiency of the dust filter. A solar cell is formed from an anode and cathode formed spiral with a semiconductor layer being laid between them to have the general form of a plate. The solar cell has, for example, a circular, a triangular or hexagonal form.