1. A method of measuring a position of a liquid surface by using a camera to take an image of the liquid surface to be measured and an image processor to process the image taken by the camera, comprising: the step of taking with the camera an image of the liquid surface, a measuring plate that is perpendicularly disposed with respect to the liquid surface, and a sub-measuring plate that is obliquely disposed with respect to the liquid surface, and the step of analyzing the image, to estimate the boundary position of the liquid surface on the measuring plate and the sub-measuring plate in order to determine the position of the liquid surface on the image.