PURPOSE: To measure fine patterns accurately by projecting the light transmitted through a reference pattern to the pattern parts to be measured of a photomask and the parts where there are light shielding materials and where there are no light shielding materials, and measuring the sizes of the patterns from the quantity of the light transmitted in the respective parts.

CONSTITUTION: Light is converted to parallel rays 10 by a lens 9 and the parallel rays are allowed to transmit through a reference pattern 18. The transmitted light 20 is projected directly to a photosensor 23, and the reference quantity of light when there are no photomasks is measured. A bright part L is moved to under the pattern 18, and the quantity of the light transmitted through the same is likewise measured with the photosensor 23. Further, the light 20 transmitted through the pattern 18 is projected to a dark part D, and the quantity of the light transmitted therethrough is measured with the photosensor 23. A photomask 21 is moved to place measuring patterns M1, M2, M3, M4 under the pattern 18, and the light 20 passed through the reference pattern is passed through the measuring patterns. The quantity of the light 22 passed therethrough is measured with the photosensor 23.