To solve the problem that, on a network for transferring a preferential packet more preferentially than a non-preferential packet, when monitoring bandwidths of preferential packets by a bandwidth monitoring device at the entry of the network while the bandwidth of preferential packets is contracted between a network user and a network provider, if the preferential packets sent by the network user have a bandwidth less than the contracted bandwidth, the network user can not sufficiently utilize the contracted bandwidth.

When a monitoring result determining section 520 detects that the bandwidths of preferential packets is less than the contracted bandwidth by the bandwidth monitoring section 141 of the bandwidth monitoring device, a differentiated service code point (DSCP) determining section 530 determines a non-preferential packet as a preferential packet. Namely, a network transmits non-preferential packets as preferential packets, so that a network user can sufficiently utilize the contracted bandwidth.

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