A data communication system and method in which a need to store a frame count value in a non-volatile memory and update the frame count value is obviated, and in which the effect resulting when a frame with its frame count value altered to a value close to a full count value is transmitted is reduced. When a valid frame count value is not held, an inquiry is made for a frame count value. The frame count value is notified from the receiving end, and the frame count value is acquired. An encryption key is generated on the basis of the frame count value. Data is encrypted with the encryption key. Data in a frame structure is transmitted. When the frame transmission ends, the frame count value is incremented.