A working machine such as a wheel loader for moving a load measures the weight of the load accurately. While the load is lifted by a boom of the working machine, a boom angle (θ) and a pressure value (P) of a boom cylinder are measured and a boom angular speed (ω) is calculated. A corrected factor (α) is determined according to the boom angular speed (ω), and a corrected pressure value (P′) is calculated from “P′=P−αω.” A predetermined table is referred to and the weight (W) of the load is determined based on the boom angle (θ) and the corrected pressure value (P) of the boom cylinder. Further, calibrations are performed as needed, and each time when a calibration is made, the average value of the calibrated value and the preceding calibrated value is calculated and data is rewritten to this average value.