To obtain a highly accurate optional calibration factor to a change in a load set over a wide range.

To plural first and second loads L1 and L2 which are formed on a span calibrating chain 23 and are mutually different in weight, an optional calibration factor K to optionally set optional load L and optional travel speed V is obtained by using first to fourth calibration factors K11, K12, K21 and K22 to plural first and second travel speeds V1 and V2. Therefore, this is formed in the constitution which uses first and second chain parts 21A and 21B different in weight or can replace weights different in weight.