PURPOSE: To obtain an optical displacement meter wherein its alarm output circuit is provided with a short-circuit and overcurrent protective function which is operated irrespective of an external input power supply and whose general purpose property is high.

CONSTITUTION: The output of a photocoupler 30 is biased by an external power supply, and drives a transistor Q1 through a resistance R3 and a diode D1. When a load is driven by the transistor Q1 as an alarm output, an electric current flows to a resistance R1 and a voltage is generated across the transistor and the resistance R1. The voltage exceeds a definite level, and the electric current flows to a base for a transistor Q2 via a resistance R2. Thereby, the electric current flows to a base for a transistor Q3 via a resistance R3, and also the transistor Q3 performs an ON operation. Thereby, the electric current which has driven the transistor Q1 primarily flows into the transistors Q2, Q3. Then, the electric current flowing via the diode D1, becomes extremely small, and the transistor Q1 loses the driving ability of the load. As a result, the transistor Q1 is protected.