An accessory apparatus detachably attachable to an image-capturing apparatus, the accessory apparatus comprising: an accessory communicator configured to provide, with the image-capturing apparatus, three channels that are a notification channel used for providing a notice from the image-capturing apparatus to the accessory apparatus, a first data communication channel used for transmitting accessory data from the accessory apparatus to the image-capturing apparatus, and a second data communication channel used for transmitting camera data from the image-capturing apparatus to the accessory apparatus, characterized by further comprising: an accessory controller configured to transmit, in response to receiving a transmission request as the notice from the image-capturing apparatus through the notification channel, the accessory data to the image-capturing apparatus through the first data communication channel, and to receive the camera data transmitted from the image-capturing apparatus through the second data communication channel, wherein the accessory communicator includes an accessory data buffer configured to store the accessory data to be transmitted to the image-capturing apparatus, the accessory controller is configured to detect a camera communication error in the camera data received from the image-capturing apparatus and is configured to, when detecting the camera communication error, produce accessory error information, transmit the accessory error information to the image-capturing apparatus and erase the accessory data stored in the accessory data buffer, and the accessory error information is accessory data including an error causing a detection of a communication error by the image-capturing apparatus when said accessory error information is received by the image-capturing apparatus. an accessory communicator configured to provide, with the image-capturing apparatus, three channels that are a notification channel used for providing a notice from the image-capturing apparatus to the accessory apparatus, a first data communication channel used for transmitting accessory data from the accessory apparatus to the image-capturing apparatus, and a second data communication channel used for transmitting camera data from the image-capturing apparatus to the accessory apparatus, characterized by further comprising: an accessory controller configured to transmit, in response to receiving a transmission request as the notice from the image-capturing apparatus through the notification channel, the accessory data to the image-capturing apparatus through the first data communication channel, and to receive the camera data transmitted from the image-capturing apparatus through the second data communication channel, wherein the accessory communicator includes an accessory data buffer configured to store the accessory data to be transmitted to the image-capturing apparatus, the accessory controller is configured to detect a camera communication error in the camera data received from the image-capturing apparatus and is configured to, when detecting the camera communication error, produce accessory error information, transmit the accessory error information to the image-capturing apparatus and erase the accessory data stored in the accessory data buffer, and the accessory error information is accessory data including an error causing a detection of a communication error by the image-capturing apparatus when said accessory error information is received by the image-capturing apparatus. an accessory controller configured to transmit, in response to receiving a transmission request as the notice from the image-capturing apparatus through the notification channel, the accessory data to the image-capturing apparatus through the first data communication channel, and to receive the camera data transmitted from the image-capturing apparatus through the second data communication channel, wherein the accessory communicator includes an accessory data buffer configured to store the accessory data to be transmitted to the image-capturing apparatus, the accessory controller is configured to detect a camera communication error in the camera data received from the image-capturing apparatus and is configured to, when detecting the camera communication error, produce accessory error information, transmit the accessory error information to the image-capturing apparatus and erase the accessory data stored in the accessory data buffer, and the accessory error information is accessory data including an error causing a detection of a communication error by the image-capturing apparatus when said accessory error information is received by the image-capturing apparatus.