PURPOSE: To wash a semiconductor substrate completely without damaging the same by irradiating the semiconductor substrate in washing with ultraviolet rays.

CONSTITUTION: A semiconductor substrate 2 to be masked is placed on an attraction plate 9 which is then housed in a washing tank 1. When the substrate 2 is irradiated with ultraviolet light 8, the light 8 is reflected or absorbed, not being transmitted through the substrate 2. The semiconductor substrate 2 is heated by part of optical energy so absorbed. Surface temperature of the substrate 2 is raised and pure water 4 in contact with the surface of the substrate 2 is also heated by thermal conduction to produce convection. The surface of the substrate 2 outside a contact hole 10 is washed by the produced convection. Additionally, no convection is produced in the contact hole 10, and only the pure water 4 in the hole 10 is boiled up to produce fine bubbles 11. Upon the fine bubbles 11 being boiled out, fresh pure water 4 enters the contact tank 10 instead of the fine bubbles 11. Thus, the inner wall of the contact hole 10 makes contact with the fresh pure water 4 each time the bubbles 11 are boiled up, whereby the interior of the contact hole 10 can be completely be washed in a short time.