A step width measurement device includes a distance calculation unit that calculates a distance between both feet of a user by using propagation data related to transmission/reception times of ultrasonic waves transmitted/received by ultrasonic transmission/reception devices mounted on each of the both feet of the user, a height calculation unit that calculates a height of a sensor by using sensor data including a spatial acceleration and a spatial angular velocity measured by sensors mounted on each of the both feet of the user, and a step width calculation unit that calculates a step width of the user by using the distance between the both feet measured at the same timing and the height of the sensor.