A power amplifying device according to an embodiment includes three or more BTL amplifiers and a closed loop section. The three or more BTL amplifiers include bridge-connected first and second output amplifiers and output a first output signal obtained by amplifying a first input signal. Switch circuits respectively corresponding to the three or more BTL amplifiers are connected in series in the closed loop section. The closed loop section is capable of forming a closed loop. An output terminal of the second output amplifier is connected to one end of the switch circuit corresponding to the second output amplifier. The switch circuit is turned on to establish connection between an output terminal of the first output amplifier and the output terminal of the second output amplifier and is turned off to break the connection between the output terminals.