A method of detecting liquid leakage from a wall portion of apparatus and/or tubes containing a liquid. A cover member is attached to an outer surface of the wall portion to form a space defined between the cover member and the wall portion, and an optical fiber is inserted into the space through a hole in the cover member. A light beam is supplied through a palarized beam splitter to the optical fiber such that reflected light from the end of the optical fiber is measured, and the presence of leakage is determined in accordance with the amount of reflected light.