A camera accessory to be detachably mounted at a camera body that includes a first body contact point, a second body contact point, a third body contact point, a fourth body contact point and a fifth body contact point and detects disengagement of the camera accessory when the fifth body contact point sustains a signal level corresponding to a first truth value continuously over a length of time equal to or greater than a predetermined time length (T 1112), the camera accessory comprising: a mounting means to be detachably mounted at the camera body; a first accessory contact point that is connected with the first body contact point while the camera accessory is engaged with the camera body so as to receive an operating voltage supplied from the camera body; a second accessory contact point that is connected with the second body contact point while the camera accessory is engaged with the camera body and to which a clock signal output from the camera body is input; a third accessory contact point that is connected with the third body contact point while the camera accessory is engaged with the camera body so as to allow a first data signal (BDAT) output from the camera body, and to which a first data signal from the camera body is input; a fourth accessory contact point that is connected with the fourth body contact point while the camera accessory is engaged with the camera body so as to allow a second data signal (LDAT) to be output to the camera body; a fifth accessory contact point that is connected with the fifth body contact point while the camera accessory is engaged with the camera body so as to allow a signal that has a level corresponding to the first truth value to be output to the camera body during a time period (T 23, T 45) in which input operation for the first data signal via the third accessory contact point and output operation for the second data signal via the fourth accessory contact point are disallowed; and an accessory-side communication control means for executing the input operation for the first data signal (BDAT) from the camera body via the third accessory contact point and the output operation for the second data signal (LDAT) to the camera body via the fourth accessory contact point in synchronization with the clock signal input at the second accessory contact point, wherein: the accessory-side communication control means is arranged to execute control so as to ensure that a time length during which the signal that has a level corresponding to the first truth value continues to be output at the fifth accessory contact point is less than the predetermined time length while the operating voltage is supplied from the camera body via the first accessory contact point. a mounting means to be detachably mounted at the camera body; a first accessory contact point that is connected with the first body contact point while the camera accessory is engaged with the camera body so as to receive an operating voltage supplied from the camera body; a second accessory contact point that is connected with the second body contact point while the camera accessory is engaged with the camera body and to which a clock signal output from the camera body is input; a third accessory contact point that is connected with the third body contact point while the camera accessory is engaged with the camera body so as to allow a first data signal (BDAT) output from the camera body, and to which a first data signal from the camera body is input; a fourth accessory contact point that is connected with the fourth body contact point while the camera accessory is engaged with the camera body so as to allow a second data signal (LDAT) to be output to the camera body; a fifth accessory contact point that is connected with the fifth body contact point while the camera accessory is engaged with the camera body so as to allow a signal that has a level corresponding to the first truth value to be output to the camera body during a time period (T 23, T 45) in which input operation for the first data signal via the third accessory contact point and output operation for the second data signal via the fourth accessory contact point are disallowed; and an accessory-side communication control means for executing the input operation for the first data signal (BDAT) from the camera body via the third accessory contact point and the output operation for the second data signal (LDAT) to the camera body via the fourth accessory contact point in synchronization with the clock signal input at the second accessory contact point, wherein: the accessory-side communication control means is arranged to execute control so as to ensure that a time length during which the signal that has a level corresponding to the first truth value continues to be output at the fifth accessory contact point is less than the predetermined time length while the operating voltage is supplied from the camera body via the first accessory contact point. the accessory-side communication control means is arranged to execute control so as to ensure that a time length during which the signal that has a level corresponding to the first truth value continues to be output at the fifth accessory contact point is less than the predetermined time length while the operating voltage is supplied from the camera body via the first accessory contact point.