An image capturing accessory detachably attachable to an image capturing apparatus, the image capturing apparatus and the image capturing accessory being respectively configured to, when the image capturing accessory is attached to the image capturing apparatus, transmit and receive command information as binary data via a contact unit of the image capturing accessory, the command information transmitted by the image capturing apparatus to the image capturing accessory being referred to as transmitting command information, the command information received by the image capturing accessory, which is transmitted from the image capturing apparatus as the transmitting command information, being referred to as received command information, the image capturing accessory comprising: a microcomputer configured to serve as: a communicator configured to communicate with the image capturing apparatus in multiple two-way communication modes; and a communication mode setter configured to set one of the multiple two-way communication modes instructed by the received command information as a use communication mode used by the microcomputer to communicate with the image capturing apparatus, wherein the microcomputer is configured to set the use communication mode corresponding to the received command information when the microcomputer determines that the transmitting command information is correctly transmitted to the image capturing accessory; characterized in that the microcomputer is further configured to: receive, from the image capturing apparatus via the contact unit of the image capturing accessory, separately from the received command information, first correspondence information as binary data, the first correspondence information being produced by the image capturing apparatus and being one's complement for the transmitting command information; use the first correspondence information when determining that the transmitting command information is correctly transmitted to the image capturing accessory; and transmit, to the image capturing apparatus, second correspondence information as binary data, the second correspondence information being produced by the image capturing accessory and being one's complement for the received command information for enabling the image capturing apparatus to check whether the transmitting command information is correctly transmitted to the image capturing accessory. a communicator configured to communicate with the image capturing apparatus in multiple two-way communication modes; and a communication mode setter configured to set one of the multiple two-way communication modes instructed by the received command information as a use communication mode used by the microcomputer to communicate with the image capturing apparatus, wherein the microcomputer is configured to set the use communication mode corresponding to the received command information when the microcomputer determines that the transmitting command information is correctly transmitted to the image capturing accessory; characterized in that the microcomputer is further configured to: receive, from the image capturing apparatus via the contact unit of the image capturing accessory, separately from the received command information, first correspondence information as binary data, the first correspondence information being produced by the image capturing apparatus and being one's complement for the transmitting command information; use the first correspondence information when determining that the transmitting command information is correctly transmitted to the image capturing accessory; and transmit, to the image capturing apparatus, second correspondence information as binary data, the second correspondence information being produced by the image capturing accessory and being one's complement for the received command information for enabling the image capturing apparatus to check whether the transmitting command information is correctly transmitted to the image capturing accessory. receive, from the image capturing apparatus via the contact unit of the image capturing accessory, separately from the received command information, first correspondence information as binary data, the first correspondence information being produced by the image capturing apparatus and being one's complement for the transmitting command information; use the first correspondence information when determining that the transmitting command information is correctly transmitted to the image capturing accessory; and transmit, to the image capturing apparatus, second correspondence information as binary data, the second correspondence information being produced by the image capturing accessory and being one's complement for the received command information for enabling the image capturing apparatus to check whether the transmitting command information is correctly transmitted to the image capturing accessory.