Provided is a respiratory muscle strengthening device, in which an inhalation start signal is generated correspondingly at an inhalation start time when a patient inhales air or an exhalation start signal is generated correspondingly at an exhalation start time when the patient exhales air, an electric signal according to the inhalation start signal or the exhalation start signal is transmitted to an electric muscle stimulator via a communication unit, a muscle pad installed on respiratory muscles of the patient enables smooth breathing motion by stimulating the respiratory muscles based on a signal transmitted from the electric muscle stimulator, and a receptor pad is provided to the respiratory muscles such that an alarm of determining whether electric stimulation is normally applied via the state of the respiratory muscles detected through the receptor pad is output, thereby securing reliability of a device.