PURPOSE: To prevent deterioration in a data transmission efficiency by using only data to be sent for an object of cyclic redundancy check and not implementing error processing even when a data part other than the check object range when a transmission capacity of a transmission line is larger than the data quantity to be sent.

CONSTITUTION: Data in 64kbps at a sender side are sent to a transmission line 7 whose transmission speed is 2Mbps. A fixed data insert section 2 inserts fixed data to a data part at the outside of an object range in data of 2Mbps based on information designating a check object range from an object range designation control section 1. A cyclic redundancy check bit generating section 3 generates and adds a check bit to the data after insertion and the resulting data are sent to the transmission line 7. A receiver side replaces similarly data at the outside of check object range into the fixed data based on the information from the transmission line 7 to eliminate a transmission error of data other than the data to be sent.