To connect and use a low pressure apparatus to a pressure reducing means for high pressure; and to prevent supply of high pressure air to the low pressure apparatus.

This adapter joint 21 for low pressure is installed in a high pressure joint 14. The adapter joint 21 for low pressure is composed of an inflow side installing port 22 installable in the high pressure joint 14, an outflow side installing port 23 for installing the low pressure apparatus and a relief valve 25 for releasing a part to outside when supplying the high pressure air. Thus, a pressure reducing valve 11 for high pressure is adjusted to the low pressure side, and a floorboard nailing machine 28 being the low pressure apparatus can be installed via the adapter joint 21 for low pressure by using the high pressure joint 14 for installing a high pressure apparatus by installing the adapter joint 21 for low pressure in the high pressure joint 14.