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 data transmission from the accessory apparatus to the image-capturing apparatus, and a second data communication channel selectively used for data transmission from the image-capturing apparatus to the accessory apparatus or data transmission from the accessory apparatus to the image-capturing apparatus; and an accessory controller configured to control data communication with the image-capturing apparatus through the accessory communicator, characterized in that: the accessory apparatus is configured to have a function of switching the second data communication channel between a first setting in which the data transmission from the image-capturing apparatus to the accessory apparatus is allowed and a second setting in which data transmission from the accessory apparatus to the image-capturing apparatus is allowed; the accessory controller is configured to transmit to the image-capturing apparatus a switching completion code related to the setting of the second data communication channel through the first data communication channel when performing a communication in the first setting after performing a communication in the second setting. 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 data transmission from the accessory apparatus to the image-capturing apparatus, and a second data communication channel selectively used for data transmission from the image-capturing apparatus to the accessory apparatus or data transmission from the accessory apparatus to the image-capturing apparatus; and an accessory controller configured to control data communication with the image-capturing apparatus through the accessory communicator, characterized in that: the accessory apparatus is configured to have a function of switching the second data communication channel between a first setting in which the data transmission from the image-capturing apparatus to the accessory apparatus is allowed and a second setting in which data transmission from the accessory apparatus to the image-capturing apparatus is allowed; the accessory controller is configured to transmit to the image-capturing apparatus a switching completion code related to the setting of the second data communication channel through the first data communication channel when performing a communication in the first setting after performing a communication in the second setting. the accessory apparatus is configured to have a function of switching the second data communication channel between a first setting in which the data transmission from the image-capturing apparatus to the accessory apparatus is allowed and a second setting in which data transmission from the accessory apparatus to the image-capturing apparatus is allowed; the accessory controller is configured to transmit to the image-capturing apparatus a switching completion code related to the setting of the second data communication channel through the first data communication channel when performing a communication in the first setting after performing a communication in the second setting.