A data communication device includes: a communication execution unit that executes serial communication of an asynchronous type; a bit variance detection block that detects a variance between bits in communication data which the communication execution unit receives from a remote communicating party; a timer that measures a low-level pulse width which is a low-level pulse duration and which is determined with the position of the variance between bits detected by the bit variance detection block; and a control unit that performs transmission speed recognizing processing on the communication data according to the low-level pulse width measured by the timer, wherein based on the confirmation that the low-level pulse width is a low-level pulse width equivalent to a bit width compatible with low-speed communication data, the control unit recognizes the transmission speed for the communication data as a low speed.