PURPOSE: To have no errors and receive no influences of a drift over a long period of time by using one electrostatic capacitance detecting means and measuring respective positions of a mask and a wafer.

CONSTITUTION: A mask 1 and a wafer 2 face to each other at a distance of a minute gap such as, for example, 40 μm, a thin conductive film 4 on the mask is electrically connected by a first probe 11, and the film 4 is grounded by a second probe 12 of the wafer 2. A calculation processing part 15 drives the ground selecting circuit 16 to ground the first prove, namely a thin conductive film 4 on the mask, and electrostatic capacitance C1 is inputted between a detecting electrode 13 and a thin conductive film 4 to be obtained from electrostatic capacitance detecting means 14. Next, the ground selecting circuit 16 is driven to insulate between the first probe 11, namely the thin conductive film 4 on the mask, and the ground, and electrostatic capacitance C2 is inputted between the detecting electrode 13 and the wafer 2 to be obtained from the electrostatic capacitance detecting means 14. Next, a gap d3 is calculated based upon an equation d3=S(1/C2-1/C1)( is a dielectric constant of an air, S is an electrode area).