A power amplifying device promptly detects an abnormal condition of a load and permits protection of an output without causing power loss. A test signal having a predetermined frequency and signal level is supplied to an amplifier, and the output level is measured. The measured level is compared with the output level that is projected when the test signal is supplied with an appropriate load connected to the amplifier so as to detect a load condition. If a determination result indicates that the load is short-circuited or released, then the operation of the amplifier is promptly interrupted.