To surely avoid 'Trap' missing, and to properly facilitate a countermeasure to plural continuous Trap missings.

A 'Set Request' analysis processing part 22 analyzes a Set Request requesting Trap re-transmission received by an SNMP protocol processing part 21 and obtains the start number and end number of the Trap group which is to be retransmitted, and instructs the Trap retransmission to the SNMP protocol processing part 21. An SNMP protocol processing part 11 detects the Trap missing, based on the comparison of the state value of 'Trap Current Number' designated by Trap or 'Get Response' from a certain SNMP agent 2 with the state value in an item data to be managed storing part 14. A Trap retransmission request processing part 13 prepares Set Request which designates the start number and end number of the Trap group to be retransmitted, in response to the Trap missing.