PROBLEM TO BE SOLVED: To measure the contact force in response to the contact force fluctuation phenomenon including a high frequency component while restricting an error.
SOLUTION: A vessel body 12 of a pantograph 10 is provided with three acceleration meters 35. Bending primary vibration mode of the vessel body 12 is grasped by the three acceleration meters 35. After grasping the vibration mode, inertia force Fina of the vessel body 12 of the pantograph 10 is found. Contact force to be applied between a trolley wire 9 and the pantograph 10 is found by subtracting the inertia force Fina from the force applied to the vessel body 12, which is found separately.