To provide a pressure control device for a transmission case, for properly controlling pressure in the transmission case while preventing mixture of foreign matters from the outside.

The transmission case 11 has an air transfer pump 12, an air holding tank 13 and a compressor 14 connected to an inner pressure regulating pipe 7 in sequence to form a closed system. Air in the tank 13 is compressed by the compressor 14. An air opening valve 15 is provided between the transmission case 11 and the pump 12. An electronic control device 18 controls the pump 12 and the compressor 14 in accordance with pressure in the case 11 detected by a pressure sensor 17. As a result, the pressure in the transmission case 11 is held in a proper range.