A mold-clamping force is detected based on an output from a strain detection apparatus (30) provided to a mold-clamping apparatus of a molding machine. A first output value output from the strain detection apparatus (30) is detected when the mold-clamping apparatus is in a maximum mold open state. A second output value output from the strain detection apparatus (30) is corrected based on the first output value when a mold-clamping force is generated by the mold-clamping apparatus. The mold-clamping force is acquired based on the corrected second output value. A correction is made after converting an output from the strain detection apparatus (30) into a digital value, or a strain gauge (50) is used so as to change a reference voltage supplied to a comparison amplifier of the strain gauge using the strain gauge based on the first output value.