A position detector for detecting the piston TDC position in a piston engine has a cylindrical casing penetrating the top wall defining a combustion chamber of the engine and fixed thereto, a light emission element, a light reception element each being accommodated within the upper half of the casing, an inlet optical fiber connected to the light emission element and an outlet optical fiber connected to the light reception element. These optical fibers are inserted into the lower half of the casing. The lower end of each optical fiber is opposed to the upper surface of the piston at a predetermined distance therefrom when the piston is located in the vicinity of the TDC position. The light emitted by the light emission element is applied to the upper surface of the piston, after passing the inlet optical fiber. The reflected light from the upper surface of the piston is received and sent to the light reception element by the outlet optical fiber. The light reception element transmits a light reception signal corresponding to the amount of the received light to the detecting circuit. The detecting circuit detects peaks of the light reception signal, which appear before and after the piston TDC position, calculates the half of the rotating angle of a crankshaft between two adjacent peaks and transmits a TDC piston detecting signal when the crankshaft rotates by the calculated rotating angle from the time the preceding peak out of two adjacent peaks appearing in the following period is detected.