A method for installing a refrigeration apparatus to a building, the refrigeration apparatus including a heat source unit, utilization-side units (12a to 12f), and a branch-off unit that is provided to a liquid-side connection pipe and a gas-side connection pipe via which the heat source unit is connected to the utilization-side units (12a to 12f), the branch-off unit including a liquid-side header provided to the liquid-side connection pipe, the liquid-side header including a single liquid-side collecting port for connection with a pipe extending to the heat source unit and liquid-side branch-off ports (54a to 54f) for connection with pipes (22a to 22f) extending to the utilization-side units (12a to 12f), the single liquid-side collecting port communicating with all of the liquid-side branch-off ports (54a to 54f), a gas-side header provided to the gas-side connection pipe, the gas-side header including a single gas-side collecting port for connection with a pipe extending to the heat source unit and gas-side branch-off ports (64a to 64f) for connection with pipes (27a to 27f) extending to the utilization-side units (12a to 12f), the single gas-side collecting port communicating with all of the gas-side branch-off ports (64a to 64f), and a casing in which the liquid-side header and the gas-side header are stored, the method comprising: an installation step of installing the heat source unit outside the building and installing the utilization-side units (12a to 12f) and the branch-off unit inside the building, characterized in that the method further comprises: a first connecting step of connecting the heat source unit and the branch-off unit to each other via a liquid-side trunk pipe and a gas-side trunk pipe, the liquid-side trunk pipe being made of a plurality of pipe members connected to each other via a mechanical pipe joint, the gas-side trunk pipe being made of a plurality of pipe members connected to each other via a mechanical pipe joint; and a second connecting step of connecting the utilization-side units (12a to 12f) and the branch-off unit to each other via liquid-side branch pipes (22a to 22f) and gas-side branch pipes (27a to 27f), each of the liquid-side branch pipes (22a to 22f) and the gas-side branch pipes (27a to 27f) being made of seamless pipes. the branch-off unit including a liquid-side header provided to the liquid-side connection pipe, the liquid-side header including a single liquid-side collecting port for connection with a pipe extending to the heat source unit and liquid-side branch-off ports (54a to 54f) for connection with pipes (22a to 22f) extending to the utilization-side units (12a to 12f), the single liquid-side collecting port communicating with all of the liquid-side branch-off ports (54a to 54f), a gas-side header provided to the gas-side connection pipe, the gas-side header including a single gas-side collecting port for connection with a pipe extending to the heat source unit and gas-side branch-off ports (64a to 64f) for connection with pipes (27a to 27f) extending to the utilization-side units (12a to 12f), the single gas-side collecting port communicating with all of the gas-side branch-off ports (64a to 64f), and a casing in which the liquid-side header and the gas-side header are stored, the method comprising: an installation step of installing the heat source unit outside the building and installing the utilization-side units (12a to 12f) and the branch-off unit inside the building, characterized in that the method further comprises: a first connecting step of connecting the heat source unit and the branch-off unit to each other via a liquid-side trunk pipe and a gas-side trunk pipe, the liquid-side trunk pipe being made of a plurality of pipe members connected to each other via a mechanical pipe joint, the gas-side trunk pipe being made of a plurality of pipe members connected to each other via a mechanical pipe joint; and a second connecting step of connecting the utilization-side units (12a to 12f) and the branch-off unit to each other via liquid-side branch pipes (22a to 22f) and gas-side branch pipes (27a to 27f), each of the liquid-side branch pipes (22a to 22f) and the gas-side branch pipes (27a to 27f) being made of seamless pipes. a first connecting step of connecting the heat source unit and the branch-off unit to each other via a liquid-side trunk pipe and a gas-side trunk pipe, the liquid-side trunk pipe being made of a plurality of pipe members connected to each other via a mechanical pipe joint, the gas-side trunk pipe being made of a plurality of pipe members connected to each other via a mechanical pipe joint; and a second connecting step of connecting the utilization-side units (12a to 12f) and the branch-off unit to each other via liquid-side branch pipes (22a to 22f) and gas-side branch pipes (27a to 27f), each of the liquid-side branch pipes (22a to 22f) and the gas-side branch pipes (27a to 27f) being made of seamless pipes.