PURPOSE: To improve the accuracy in the positional detection by forming a round sweeping position marker on the CRT of a spiral sweep type detector.

CONSTITUTION: A CRT is subjected to the spiral sweep with sine and cosine waves which are modulated by the saw-tooth waves of an integrating circuit 4. On the other hand, a counter 14 counts the sine waves 9 to generate a plural pulses C during one period of the saw-tooth waves. During this generation, the integrating operations of the circuit 4 are stopped, and at the same time the grid of the CRT is controlled to effect the display of a bright spot. During the generation of the pulses C, on the other hand, switches SW2 to SW5 are interchanged to supply the X and Y axis deflecting circuits of the CRT with the sine and cosine waves, which have their amplitudes modulated by an amplitude voltage setting unit 19, and with X and Y axis deflecting voltages, thus forming a round bright line Ma. The size of the line Ma is determined by the set voltage of the setting unit, and the pulses of a test piece can be displayed at the center of the line Ma by the operations of the deflecting voltages 17 and 18.