PURPOSE: To make a quantitative check on whether various digital transmission errors are processed correctly, by generating the digital transmission errors of random polarity and length at a random error generation rate.

CONSTITUTION: Error-generation-rate assigning circuit 8 assigns the generation interval of error data consisting of one bit or a few bits. Error-bit-length counter 9 counts the bit length of one burst error. Error-length assigning circuit 10 assigns the bit length of one burst error. Error-polarity assigning circuit 11 assigns by a switches, etc., one of the holding of transmitted data in a mark, the holding of transmitted data in a space and the inversion of transmitted data.