Abstract: An apparatus for photochemically decomposing a photoresist is disclosed wherein processing time can be minimized by inducing a uniform photochemical oxidative decomposing reaction over the whole surface of a base board, and an ozone gas atmosphere can be utilized efficiently. The apparatus includes a rotatable circular table supporting one or more base boards and an ashing platform arranged around the periphery of the table including an ozone feeding slit and an ozone discharge slit. The apparatus also includes a plurality of ultraviolet ray lamps arranged inside a box-shaped vessel located above the ashing platform. An upper cover plate is pivotally mounted on the ashing platform and includes a transparent window plate through which the ultraviolet rays pass to irradiate the base boards. An airtight flat space is formed between the ashing platform and the upper cover to contain the ozone atmosphere.