A method for acquiring a packet of a monitoring target, the method includes, forming, by a control unit, a mirror path for arriving a mirror packet, of which a origin or a destination of a transmission is a virtual machine for the monitoring target, at a monitoring device which monitors the mirror packet through one or more relay devices, detecting, by the control unit, an occupation when a packet except the packet, of which the origin or the destination of a transmission is the virtual machine for the monitoring target, does not pass based on history information of the packet which passes the relay device, accumulating, by the monitoring device, the mirror packet which passes the relay device, and extracting, by the monitoring device, the mirror packet which arrived at in the occupation as the packet for the monitoring target.