A wearable environmental sensor device includes a temperature/humidity sensor disposed on a wall surface of a housing that is exposed to an environment and configured to measure ambient environmental information around a living body, and a protective structure formed around the temperature/humidity sensor. The temperature/humidity sensor is disposed, directly or via a support structure, on or over the wall surface of the housing, wherein the wall surface faces substantially downward from the living body when the wearable environmental sensor device is attached to the living body and the living body is in a standing posture. The protective structure has respective ventilation holes provided in two or more pairs of opposed surfaces thereof each facing in a direction other than a vertical direction of the living body when the wearable environmental sensor device is attached to the living body and the living body is in the standing posture.