To perform the error detection of transfer data by calculating the sum of logical values of respective bits for each byte of a parallel data signal through a control line and making the control line active when this value is an odd number but making the control line in active when this value is an even number.

A parity calculation part 11 calculates a parity value based on the transmission data of one byte. This parity value is found by extracting the respective bits of data of one byte to be transmitted and calculating the value of sum of these respective bits. Corresponding to this found parity value, an EOI line is determined and according to this determination, an EOI line control part 12 judges whether the parity value is the odd number or the even number. When that judgement shows the odd number, an EOI line 4 is made active but when it shows the even number, the EOI line 4 is made inactive. After end at the EOI line control part 12, data are transmitted onto a data bus 3 by a data transmission part 13. Thus, the error of transfer data can be surely detected.