To make the input of a speed power generator and the detection of fault occurrence highly reliable by obtaining an output signal to be turned on/off based on the current change of a current detecting element which is generated in accordance with the voltage change of a signal voltage generating source.

The speed power generator 21 to be the object of waveform shaping is inserted to a shunt circuit on a diode 15 side and a photocoupler 11 for detecting its signal change is inserted to the shunt circuit on the diode 16 side. When the output voltage VTG of the speed power generator 21 is zero (train stop state), the input current ID of the photocoupler 11 turns on its output transistor and keeps a state where the final output TGOUT of a waveform shaping circuit 1 is one through a Shumitt trigger inverter 41. When the output voltage VTG of the speed power generator 21 is increased with the acceleration of a train and a voltage becomes the one for turning-off the output transistor of the photocoupler 11, a pulse signal with a frequency being equal to the output frequency of the speed power generator 21 appears in the final output TGOUT.