PURPOSE: To improve the detecting accuracy by using a power source for a spot light source, which rises in response to any command signal to monotonously increase when the output current of a position locating element reaches a preset value.

CONSTITUTION: Simultaneously as an integrating device 6 is reset in response to the pulses from a clock pulse generator 7, a pulse generator is set to raise the pulses from a terminal Q. Then, a lamp voltage is fed from the integrating device 6 through a switch 81 to an amplifier 91 so that the amplifier 91 feeds an LED 101 with a current corresponding to the input voltage to monotonously increase its brightness. Since the output current of a position detecting element 1 is also monotonously increased in accordance therewith, this current is converted into a voltage which is fed to a comparator 4. When a standard voltage is reached, pulses are fed to the terminal R of a generator 5 and to the control terminal of a switch 32 thereby to break the pulses from the terminal R. Then, the output of the device 6 stops its increase at that level to hold the maximum brightness of the LED 101. Thus, it is possible to enhance the detecting accuracy while eliminating the distortion in the position.