An optical measuring device capable of improving the precision of detection of contamination on a protective glass is disclosed. With threshold values as large as 90%, 50%, 10% of a peak value of a scan signal, measured values Q.sub.90, Q.sub.50, Q.sub.10 of a dimension of a work to be measured are derived, respectively. It is determined whether a difference between the measured value Q.sub.90 and the measured value Q.sub.50 (or a variation of the measured value) is equal to or larger than a predetermined reference value. (B)-(D) show the presence of contamination on the protective glass, equal to or larger than the reference value. (A) shows the absence of contamination, lower than the reference value. It is then determined whether a difference between the measured value Q.sub.50 and the measured value Q.sub.10 is equal to or larger than a predetermined reference value. (D) shows the presence of thick contamination on the protective glass, equal to or larger than this reference value. Accordingly, a buzzer is driven and a red lamp is turned on to give the alarm to the operator to remove the contamination immediately.