Abstract: An alarm device is designed to warn of danger of hitting high voltage overhead power lines by a crane in motion. The alarm device is composed of an electric-field sensor for detecting the alternating electric field of the overhead power lines and for outputting an alternating current, which is then converted or amplified by an amplifier connected with the electric-field sensor. An alternating voltage is outputted by the amplifier such that the alternating voltage is rectified and filtered by a rectifier/filter circuit connected with the amplifier. A direct current voltage output is brought about by the rectifier/filter circuit. A starter is disposed between the rectifier/filter circuit and a wireless transmitter capable of being activated by the starter to transmit a wireless signal at such time when the value of the direct current voltage exceeds a preset value. The alarm device is further composed of a wireless receiver/alarm for receiving the wireless signal and for effecting a warning signal.