A wireless sensor device comprising: a flexible circuit board; at least one sensor selected from the group consisting of a temperature sensor, a humidity sensor, an illuminance sensor, and other sensors that sense various environmental conditions; a controller that processes an output value received from the sensor; an antenna that wirelessly transmits a signal received from the controller; a replaceable primary battery; a first adhesive layer; a flexible sheet; a removable second adhesive layer between the flexible circuit board and the flexible sheet; and a protective layer that protects the at least one sensor, the controller, the antenna, and the primary battery; wherein the at least one sensor, the controller, the antenna, and the primary battery are on the flexible circuit board, wherein (a) the first adhesive layer, the flexible circuit board, and the flexible sheet are stacked together in this order, or (b) the first adhesive layer, the flexible sheet, and the flexible circuit board are stacked together in this order, wherein the at least one sensor, the controller, the antenna, and the primary battery are disposed between the flexible circuit board and the flexible sheet, wherein the flexible circuit board and the flexible sheet form a casing of the wireless sensor device, wherein the protective layer is disposed between the flexible circuit board and the second adhesive layer, and wherein the protective layer and the second adhesive layer are hollowed to form a space housing the primary battery. a flexible circuit board; at least one sensor selected from the group consisting of a temperature sensor, a humidity sensor, an illuminance sensor, and other sensors that sense various environmental conditions; a controller that processes an output value received from the sensor; an antenna that wirelessly transmits a signal received from the controller; a replaceable primary battery; a first adhesive layer; a flexible sheet; a removable second adhesive layer between the flexible circuit board and the flexible sheet; and a protective layer that protects the at least one sensor, the controller, the antenna, and the primary battery; wherein the at least one sensor, the controller, the antenna, and the primary battery are on the flexible circuit board, wherein (a) the first adhesive layer, the flexible circuit board, and the flexible sheet are stacked together in this order, or (b) the first adhesive layer, the flexible sheet, and the flexible circuit board are stacked together in this order, (a) the first adhesive layer, the flexible circuit board, and the flexible sheet are stacked together in this order, or (b) the first adhesive layer, the flexible sheet, and the flexible circuit board are stacked together in this order, wherein the at least one sensor, the controller, the antenna, and the primary battery are disposed between the flexible circuit board and the flexible sheet, wherein the flexible circuit board and the flexible sheet form a casing of the wireless sensor device, wherein the protective layer is disposed between the flexible circuit board and the second adhesive layer, and wherein the protective layer and the second adhesive layer are hollowed to form a space housing the primary battery.