To provide an inexpensive non-contact type highly accurate measuring system by solving the following problems: in perpendicularity measurement for long size such as a mask substrate for a large size liquid crystal, a large size machine processing face, and a large size level block reference face, a laser reflection type collimetor or the like is used, and an ultra-precision drive reference axis is especially expensive in long measurement exceeding 1 m in a large mask substrate, while a problem of contact with an object to be measured by using a level or the collimetor occurs.

Increase of accumulated errors and restriction of measurement pitches are improved in multi-point measurement being defects of the conventional successive three-point measurement method. The accumulated errors of the successive three-point measurement is proportional to a square of the number of measurement points. In the invention, a long entire length is divided into several blocks, parts between the blocks are measured in the conventional three-point method with three sensors 21, 22 and 23. The inside of the block is measured by simultaneous signal processing with a fourth sensor 24, and remarkable reduction of the accumulated errors and option of the measurement pitches are enabled.

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