To enable anyone to grasp the force to a vehicle frame easily at any time and at any place by calculating the difference of acceleration information in upper and lower directions from each acceleration detector being provided at the front and rear parts of a vehicle.

When acceleration information S1 and S2 from first and second acceleration detectors 151 and 152 generate a difference ΔS, the difference ΔS is caused by the mass difference between the positions of a main frames 12 where the first and second acceleration detectors 151 and 152 are placed. More specifically, a result being obtained by multiplying a mass at the side of the main frame 12 where the first acceleration detector 151 is provided by the difference ΔS becomes a force to the main frame 12. The similar is applied also to the second acceleration detector 152. Also, the force being applied to the main frame 12 that is obtained as a result of calculation is equivalent to a pitching force FP. Then, since a mass itself has a constant value, the difference ΔS is calculated and the difference ΔS is monitored, thus enabling anyone to grasp the pitching force FP easily at any time and at any place.