A Roberval's mechanism and a lever are configured separately. To the Roberval's mechanism, a lever housing section is formed for inserting the lever in the longitudinal direction and enabling the lever to be located between a pair of the upper and the lower Roberval's sections. To the other end of the lever, a section extending outwards beyond the lever housing section is equipped. A balance driving portion are mounted to the section equipped to the other end of the lever and extending outwards. The lever block is situated to extend through a hollowed out section of the fixed end of the Roberval load block. During manufacture, the lever is formed from a separate piece that has temporary bridges formed to maintain the fixed section and the mobile sections relatively immobile so that the spring sections are protected during assembly. After the lever is connected to the Roberval mechanism, the bridges are cut.