A camera to which a plurality of accessory devices is mountable, the camera comprising: a camera control unit configured to control communication that is performed between the camera and all the accessory devices mounted to the camera via channels, the channels including a notification channel (CS) used for signal transfer and a data communication channel (DATA) used for data communication, and a power supply unit configured to supply electric power to all the accessory devices mounted to the camera , wherein all the accessory devices mounted to the camera and the camera are each able to set the notification channel (CS) to first setting and second setting, wherein a voltage level of the notification channel (CS) takes a first level and a second level, wherein the first level corresponds to the first setting when any of the camera and all the accessory devices mounted to the camera sets the notification channel (CS) to the first setting, wherein the second level, which is different from the first level, corresponds to the second setting when each of the camera and all the accessory devices mounted to the camera sets the notification channel (CS) to the second setting, and wherein the camera control unit starts the data communication via the data communication channel (DATA) with all the accessory devices mounted to the camera on condition that the voltage level of the notification channel (CS) takes the second level in a state in which the camera control unit sets the notification channel (CS) to the second setting after the power supply unit starts to supply electric power to all the accessory devices mounted to the camera. a camera control unit configured to control communication that is performed between the camera and all the accessory devices mounted to the camera via channels, the channels including a notification channel (CS) used for signal transfer and a data communication channel (DATA) used for data communication, and a power supply unit configured to supply electric power to all the accessory devices mounted to the camera , wherein all the accessory devices mounted to the camera and the camera are each able to set the notification channel (CS) to first setting and second setting, wherein a voltage level of the notification channel (CS) takes a first level and a second level, wherein the first level corresponds to the first setting when any of the camera and all the accessory devices mounted to the camera sets the notification channel (CS) to the first setting, wherein the second level, which is different from the first level, corresponds to the second setting when each of the camera and all the accessory devices mounted to the camera sets the notification channel (CS) to the second setting, and wherein the camera control unit starts the data communication via the data communication channel (DATA) with all the accessory devices mounted to the camera on condition that the voltage level of the notification channel (CS) takes the second level in a state in which the camera control unit sets the notification channel (CS) to the second setting after the power supply unit starts to supply electric power to all the accessory devices mounted to the camera. <EOS>