PURPOSE: To relieve the software load by devising the system such that an error is not detected at a checked location but a monitor cell check section in a monitor package sends and checks a monitored cell in the equipment definitely.

CONSTITUTION: Each monitor cell check section 11a provided to an incoming side and an outgoing side in 1st package 11-1 to n-th package 11-n checks respectively a monitored cell sent from a monitor cell transmission section 10a or a monitored cell transmission section 11b of a pre-stage in a monitor package 10 at a time interval of T0. When no monitored cell comes even after lapse of T0 or over, it is taken as a time-over error and each error information is inserted to the monitored cell and the result is sent to a transmission section 11b of a next stage. Furthermore, each transmission section 11b provided to the incoming side and the outgoing side in the 1st package 11-1 to n-th package 11-n checks respectively the presence of an error occurred in the package 11-1 to n-th package 11-n and in the case of the check error, each error information is inserted to the monitored cell and the result is sent to the check section 11a of the next stage.