A relief-valve device for an air pressure gauge has a casing, a relief-valve assembly, a valve shaft, and a push button. The casing has a connecting member, a pressure detecting member and a discharging member. The relief-valve assembly is mounted in the discharging member and has a valve seat, a first seal ring, a second seal ring and a mounting cover. The valve shaft is mounted axially slidably in the relief-valve assembly and has a shaft member and a valve head. The shaft member has a discharging notch selectively aligning with the second seal ring to allow high pressure air in an inflatable product connecting to the air pressure gauge to be discharged slowly and finely out of the relief-valve device.