To measure loaded weight in respective work statuses of a dump truck. The loaded weight of a dump truck 11 is calculated and stored at prescribed time intervals. It is judged whether or not the dump truck 11 has transferred to a transporting status (S31-S33), and if it has transferred to a transporting status (S31: YES, S32: YES, S33: YES), then the transported weight G1 at the end of loading and immediately before transition to the transporting status is calculated and stored (S34). The transported weight G2 during the period in which the truck is in the transporting status is also calculated and stored (S35). If the truck has transferred from the transporting status to an unloading status (S36: YES), then the transported weight G3 before unloading is calculated and stored (S37). A transported weight selected from these respective transported weights G1-G3 is read out and output to a display device, or the like (S38, S39). Thereby, it is possible to measure the transported weight at a previously established plurality of measurement timings.