An activity information meter is provided which is capable of stable transmission between a measuring device and an output device and is highly convenient. In a measuring device 1 or 101 and an output device 51 or 151, a selector is placed between a current output section and each electrode and between a voltage detection section and at least one electrode and a transmission route is selected as appropriate, thereby making possible bidirectional transmission using a body as a medium, a change in potential is detected based on a reference potential at the time of reception without using the earth as a reference potential or a voltage generated based on a body part impedance is detected without using the earth as a reference potential. As a result, the activity information meter is hardly influenced by the environment.