A tire condition monitor device includes a circuit board mounted on a tire wheel for detecting a pneumatic pressure in an interior of a tire. The circuit board is covered with urethane resin. The urethane resin has finer resin meshes as compared with the conventional silicon resin serving as a board protecting resin. Since sulfur gas and water is hard to permeate through the urethane resin, gas barrier and dampproof properties can be improved. In a method of manufacturing the tire condition monitor device, a prebaking process is carried out before a board protecting process of covering the circuit board with the urethane resin. Since moisture contained in the circuit board is vaporized, the circuit board covered with the urethane resin can reliably be protected from moisture.