An activity meter includes a sensor for detecting acceleration in a plurality of directions, representative acceleration calculation means for calculating a representative acceleration based on an output signal of the sensor, and exercise intensity calculation means for calculating an exercise intensity of a body motion performed in the unit period from the representative acceleration. The exercise intensity calculation means includes a plurality of calculation formulas to use in the calculation of the exercise intensity, and determines the calculation formula to use in the calculation of the exercise intensity of the body motion performed in the unit period based on a horizontal component and a vertical component of the representative acceleration. The activity amount on various types of body activities from daily activity to exercise thus can be accurately measured.