To provide a monitor and a monitoring method capable of accurately detecting a failure in a substrate to be controlled even in during access.

This monitor 10 supplies an output signal 124 from an address decoder 12 to an access period monitoring part 14 and a recognition response output circuit 20, the access period monitoring part 14 represents the period between an access start and the reception of an ACK signal with an output signal 148, a state monitoring circuit 18 outputs an output signal 132 corresponding to mounting information 130, and a state recognition circuit 16 supplies a recognition signal 150 showing whether to be in a prescribed state to the output circuit 20 from the output signals 148 and 132, generates an access extraction ACK signal 152 according to the recognition signal 150 and the output signal 124 and outputs the access extraction ACK signal 152, thereby notifies a mounted device of what state the substrate of a controlling part 32 to be transmitted is in during access.