PURPOSE: To improve the reliability of a communication system using a fixed- length packet by inserting continuous pseudo random signals into the data pets of the continuous packets so as to transmit them on a transmission side and judging the number of abolished packets generated between the two packets on a reception side.

CONSTITUTION: On the transmission side, the pseudo random signals are inserted into the data part of the packet so that it succeeds to the data part of the preceding packet in a pseudo random signal insertion part 4. On the reception side, the pseudo random signals inserted into the data parts of the two continuous packets are stored in a PN pattern storage part 1, stored pseudo random patterns are combined and it is set to be an address given to a judgement part 2. The judgement part 2 reads the number of the abolished packets corresponding to the pattern of the address from a table and judges the abolished packet. Thus, the number of the abolished packets can precisely be judged in spite of the number of the abolished packets without narrowing an area into which test information is inserted.