A network monitor for an in-vehicle network used in a vehicle, the in-vehicle network being formed by multiple electronic control units including a source unit, a destination unit, and one or more repeaters (100, 100-1, 100-2, 100-N) that form a communication path in the in-vehicle network, each of the one or more repeaters (100, 100-1, 100-2, 100-N) including an implementation of the network monitor, the network monitor comprising: a receiver that receives an announcement transmitted by the source unit using a mechanism for bandwidth reservation, the announcement including a value of a first bandwidth for the source unit to perform first data communication; a database that holds a white list in which a second bandwidth is specified on a data communication type basis, wherein the white list includes parameters, on a data communication type basis, indicating a normal band value and an abnormality judgment value that specify the second bandwidth; a determiner that determines whether to reserve the first bandwidth by comparing the value of the first bandwidth with the second bandwidth for the first data communication specified in the white list, wherein the determiner compares the value of the first bandwidth with the parameters included in the white list and determines whether to reserve the first bandwidth depending on whether the value of the first bandwidth is included in a range of the second bandwidth specified by the normal band value and the abnormality judgment value; a bandwidth reserver that reserves the first bandwidth depending on a determination result of the determiner; and a transmitter that transmits the determination result of the determiner. the in-vehicle network being formed by multiple electronic control units including a source unit, a destination unit, and one or more repeaters (100, 100-1, 100-2, 100-N) that form a communication path in the in-vehicle network, each of the one or more repeaters (100, 100-1, 100-2, 100-N) including an implementation of the network monitor, the network monitor comprising: a receiver that receives an announcement transmitted by the source unit using a mechanism for bandwidth reservation, the announcement including a value of a first bandwidth for the source unit to perform first data communication; a database that holds a white list in which a second bandwidth is specified on a data communication type basis, wherein the white list includes parameters, on a data communication type basis, indicating a normal band value and an abnormality judgment value that specify the second bandwidth; a determiner that determines whether to reserve the first bandwidth by comparing the value of the first bandwidth with the second bandwidth for the first data communication specified in the white list, wherein the determiner compares the value of the first bandwidth with the parameters included in the white list and determines whether to reserve the first bandwidth depending on whether the value of the first bandwidth is included in a range of the second bandwidth specified by the normal band value and the abnormality judgment value; a bandwidth reserver that reserves the first bandwidth depending on a determination result of the determiner; and a transmitter that transmits the determination result of the determiner. a receiver that receives an announcement transmitted by the source unit using a mechanism for bandwidth reservation, the announcement including a value of a first bandwidth for the source unit to perform first data communication; a database that holds a white list in which a second bandwidth is specified on a data communication type basis, wherein the white list includes parameters, on a data communication type basis, indicating a normal band value and an abnormality judgment value that specify the second bandwidth; a determiner that determines whether to reserve the first bandwidth by comparing the value of the first bandwidth with the second bandwidth for the first data communication specified in the white list, wherein the determiner compares the value of the first bandwidth with the parameters included in the white list and determines whether to reserve the first bandwidth depending on whether the value of the first bandwidth is included in a range of the second bandwidth specified by the normal band value and the abnormality judgment value; a bandwidth reserver that reserves the first bandwidth depending on a determination result of the determiner; and a transmitter that transmits the determination result of the determiner.