In a method of surface treatment irradiating a part of a plate to be treated with light flux, detecting a quantity of transmitted light or reflected light on the plate corresponding to the light flux, detecting an end point of surface treatment based on the variation in the detected signal level the combination with the method of a step of detecting whether the detected signal level exceeds a predetermined target level within a predetermined period of time, a step of repeatedly setting up a new target level so long as the detected level is found exceeding the predetermined target level, and a step of determining the end point of the surface treatment at the moment when the detected signal level is found not exceeding the new target level.