To provide a no-load detection protecting device for a neon inverter transformer, sure and economical with a very simple constitution, by detecting its secondary side in a no-load condition and preventing an accident or the like previously by reducing or interrupting an output.

A commercial power supply 1, which is an AC power supply, is converted into DC power by a rectifier circuit 2, and in a side of a primary coil 3, by alternately switching transistors Tr1, Tr2 by a prescribed frequency, high frequency high voltage AC power is generated in a side of a secondary coil 5 to be supplied to an electrode of a neon tube 6. In the case of generating a reverse current I2 from the primary coil 3 in a point X after rectifying the AC power supply of the primary side, gates of the transistors Tr1, Tr2 are earthed through diodes D1, D2 and a thyristor SCR1 connected in series in a reverse direction to the gates of the transistors Tr1, Tr2.