Network service server load balancing

For use with a system in which data is transferred from a first control unit (5-1) to a plurality of second control units (1-1, 1-2, ..., 1-m) over a network (2), a network load balancing device (3) comprises: a conversion information storage unit (4-a) which stores conversion information representing a correspondence relationship of the first control unit (5-1) to one of the second control units (1-1, 1-2, ..., 1-m); a distributive relay unit (4) which transfers data sent by the first control unit (5-1) to one or more of the second control units (1-1, 1-2, ..., 1-m) on the basis of the correspondence relationship stored in the conversion information storage unit (4-a); a distribution ratio adjusting unit (7) which adjusts the correspondence relationship in the conversion information stored in the conversion information storage unit (4-a); and a load measuring unit (6) which measures the load on each of the second control units (1-1, 1-2, ..., 1-m) and notifies the distribution ratio adjusting unit (7) of the measurements to allow the unit to adjust the correspondence relationship in the conversion information stored in the conversion information storage unit (4-a).