To improve measurement accuracy of a road gradient.

When a GPS 12 has receiving sensitivity equal to or higher than a specified level and a change in an altitude is detected, a gradient calculation unit 25 calculates a road gradient using data on a running speed and an altitude obtained from the GPS 12 through a GPS data acquisition unit 21. When there is no change in an altitude, a constant travel resistance calculation unit 22 calculates a constant travel resistance using data obtained through an ECU data acquisition unit 23 with a road gradient of 0 and a calculation result is held in a constant travel resistance holding unit 27. When the receiving sensitivity of the GPS 12 is lower than a specified level, the gradient calculation unit 25 calculates a road gradient using the constant travel resistance held in the constant travel resistance holding unit 27 and the ECU data obtained through the ECU data acquisition unit 23.