PURPOSE: To easily and properly diagnose a deteriorating condition of tongs by previously obtaining a relation between grasping force of the tongs and a wearing amount in a connecting part and measuring this grasping force.

CONSTITUTION: When a grasping force detecting bar 30 is held by a pair of lower part arms 22 of tongs, grasping force is caught. This grasping force detecting bar 30 is provided with a bar main unit 31 and a pair of detecting parts 32 provided in both ends of the main unit 31. The detecting part 32 is provided with a protecting member 33, protrusion part 34 and a strain gage provided therein, and the grasping force is detected by pressing the protrusion part 34 through the protecting member 33 and transmitting pressing force of the protrusion part 34 to the strain gage 35 when the grasping force detecting bar 30 is held by the lower part arms 22. This grasping force is decreased in accordance with increase of a looseness amount, that is, play amount of a holding arm 20. Looseness is generated by wear so as to correspond to the maximum clearance quantity between a pin hole and a pin or a bush.