PURPOSE: To provide a method and device for precisely and quickly measuring the mean thickness of a plate-shaped object to be measured by forming air films on the two surfaces of the object.

CONSTITUTION: An jetting plate 2 having air jets over the surface is approached to each surface of a plate-shaped object to be inspected 1. The air film thicknesses on both sides are judged from the spacing of the air jetting surfaces of the two plates 2, air supply amount per unit time and unit area, the weight of the object 1, and the weight of the upper plate 2 and its accessories, and thereupon the mean thickness of the object 1 is determined.