To secure a stable radio-communication performance in a sensor node, the sensor node with a radio-communication circuit and a sensor, for transmitting data measured by the sensor through radio-communication, includes a first board BO2 on which an antenna ANT1 connected to the radio-communication circuit is placed, a case CASE1 containing the first board BO2, and a band that is attached to the case CASE1 so as to fix the case CASE1 to the skin. The antenna ANT1 is placed in an upper portion of the case CASE1, which corresponds to a 12 o'clock direction of a wristwatch.