PURPOSE: To measure easily and accurately the size of a real mold clamping force, by a method wherein a compression-deforming quantity of one like constituting a single toggle mechanism is detected at a nondividing part where the force is transmitted without being divided.

CONSTITUTION: A mold is set up between a stationary platen 2 and movable platen 3, zero point adjustment of a strain gauge 16 is performed in a measuring instrument 19 under a mold breaking state and then a mold clamping actuation is performed. With this construction, since a driving side crank pin 12 is driven by a servomotor through a reduction gear 14, a driven side of the second like 9 is extruded through the first like 8 and a connecting crank pin 10, the movable platen 3 is moved forward through a driven side crank pin 11 and becomes a mold contacting state. Furthermore, when a servomoter 13 is driven, the mold is turned into a lock-lup state where the mold is press-bonded with the force commensurate with its extension. In this instance, since a nondividing part 17 of the second like 9 is compressed by receiving a mold clamping force, the mold clamping force is calculated by detecting its deforming quantity.