To prevent the erroneous conveyance, etc. of a substrate by detecting a substrate stored in an incorrect posture in a cassette.

A sensor support arm 13 is moved up and down relative to the cassette C placed on a cassette placement table 1. The sensor support arm 13 is mounted with first and second wafer sensors 11 and 12 which are located symmetrical with respect to the center line of a wafer. When moving up and down the sensor support arm 13 by an elevation mechanism 20, if the wafer W is held in a correct posture in the cassette C, output signals from the first and second wafer sensors 11 and 12 are consistent with each other. However, if the wafer W is held aslant in the cassette C, the output signals of the first and second wafer sensors 11 and 12 are not consisted with each other.