In a surface treatment apparatus, an organic substance is removed through ashing by ultraviolet rays and ozone at atmospheric pressure. A transparent partition plate is provided in closely spaced, opposed relation to a material to be treated, and at least one gas outlet is open to that surface of the partition plate opposed to the material to be treated. With this arrangement, the gas can be directly collected via the gas outlet during the treatment, and the end point of the organic substance removal treatment can be detected by analyzing the collected gas.