To utilize optics for solving problems in a conventional bubble-tube level gauge that it has visual recognition error owing to bubble expansion caused by atmospheric temperature and visual recognition is difficult in a dark place.

A plurality of conical light beams 9 are applied to a spherical mirror-finished surface 3 rolling on a recessedly curved surface 2 according to the slope of a housing 1 to project their reflected light beams 10 onto a screen 6. The slope of the housing 1 can be measured with high accuracy and with high sensitivity by visually recognizing the differences of a projected circle from a center point marker 7 and from a peripheral circle marker 8 in shape and gap. Visual recognition measurement in a dark place is possible since a light emission source 4 is provided.