PURPOSE: To attain sure data transmission by returning serial data to a transmission side every time one bit of the data is received and applying correcting processing while it is discriminated as generation of a transmission error when the content of the returned data is dissident with the content of the transmission data.

CONSTITUTION: When the 1st unit 1 is set to the transmission mode, a start bit SB inverted at the middle of one bit from "L" to "H" is transmitted. Then the data of the 1st bit is transmitted from the 1st unit 1. On the other hand, the 2nd unit 2 is set to the reception mode, and when the "L" level part of the start bit SB transmitted from the unit 1 is received via a signal line 3, the unit 2 starts a timer counting one bit time in synchronization with the inversion of the midpoint of the bit SB from "L" to "H". Furthermore, the data is fetched by sampling the reception signal in the timing when the timer completes the time count and returned to the 1st unit 1 over one bit period via a signal line 4.