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 (RTS) used for providing a notice from the image-capturing apparatus to the accessory apparatus, a first data communication channel (DLC) used for data transmission from the accessory apparatus to the image-capturing apparatus, and a second data communication channel (DCL) used for data transmission from the image-capturing apparatus to the accessory apparatus; and an accessory controller configured to control data communication with the image-capturing apparatus through the accessory communicator, characterized in that : the image-capturing apparatus and the accessory apparatus are each 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, in response to receiving from the image-capturing apparatus through the notification channel a first notice indicating completion of switching from the first setting to the second setting in the image-capturing apparatus, switch from the first setting to the second setting in the accessory apparatus. an accessory communicator configured to provide, with the image-capturing apparatus, three channels that are a notification channel (RTS) used for providing a notice from the image-capturing apparatus to the accessory apparatus, a first data communication channel (DLC) used for data transmission from the accessory apparatus to the image-capturing apparatus, and a second data communication channel (DCL) used for data transmission from the image-capturing apparatus to the accessory apparatus; and an accessory controller configured to control data communication with the image-capturing apparatus through the accessory communicator, characterized in that : the image-capturing apparatus and the accessory apparatus are each 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, in response to receiving from the image-capturing apparatus through the notification channel a first notice indicating completion of switching from the first setting to the second setting in the image-capturing apparatus, switch from the first setting to the second setting in the accessory apparatus. the image-capturing apparatus and the accessory apparatus are each 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, in response to receiving from the image-capturing apparatus through the notification channel a first notice indicating completion of switching from the first setting to the second setting in the image-capturing apparatus, switch from the first setting to the second setting in the accessory apparatus. <EOS>