Main objective of this study is to evaluate physical activity by means of a portable device that estimates the type of physical activity and the exercise intensity. A simple and effective algorithm was proposed to classify physical activities including vertical movements, such as stair climbing or use of elevator. A portable measurement device with accelerometers and a barometer was specially designed to detect the features of vertical movements. Walking speed was calculated by an equation which estimates the walking speed as a function of signal energy of vertical body acceleration during walking. Healthy young subjects participated in our experimental study. The portable device was attached to the waist. To confirm the feasibility of the method, oxygen uptake was also measured by a portable expiratory gas analyzer. Experimental results showed that the proposed method classified all type of ambulatory defined in the algorithm successfully. Resultant energy consumption estimated by the proposed method was found to be similar to the measured values of the expiratory gas analyzer. It was suggested that the consideration of vertical movements made a improvement in the estimation of energy consumptions, and the proposed method provides better estimation of physical activity.