A device for measuring the concentration of a gas that makes up ambient air comprises a wristwatch comprising a watch case (13) containing a watch movement, time indicating members (1, 3, 9) visible through a crystal of the watch case (13). The wristwatch furthermore comprises at least one sensor (11) designed to measure the concentration of a gas involved in the makeup of the ambient air. The wristwatch also comprises means for actuating an alarm when the gas concentration measured by the sensor (11) reaches or crosses a threshold. The device has two operating states: a "time" state and an "alarm" state, in which states, in the "time" state, the time-indicating members (1, 1 ', 3, 9) indicate time and the sensor (11) constantly measures the concentration of gas without displaying it or activating the alarm. If the sensor (11) detects a concentration of a gas, the device switches into "alarm" state in which the indicator member (4, 4', 8, 9, 10) displays this measured value and/or the means for actuating an alarm actuate an alarm normally without the time being displayed.