A method for installing a spray pipe for injecting a liquid absorbent into an absorption tower which is formed to extend in a vertical direction and serves as a passage for exhaust gas, the spray pipe including a cylindrical pipe portion extending along an axis line and including a closed leading end, a plurality of nozzle holders which are disposed at a plurality of locations of an upper end portion in the vertical direction of the pipe portion and configured to guide the liquid absorbent flowing through the pipe portion in the horizontal direction, upward in the vertical direction, a leg portion disposed on a lower end portion in a vertical direction of the pipe portion and including an installation surface, and an attachment flange disposed at a side of a base end of the pipe portion and including a first attachment surface, and the absorption tower including an opening portion opened toward a side of absorption tower, a second attachment surface disposed at the opening portion, and a supporting portion (91,92,93) including a supporting surface (91a,92a,93a) supporting the pipe portion, the method comprising: an insertion step of inserting the spray pipe from the opening portion; a positioning step of positioning the spray pipe such that a state is established in which the installation surface of the leg portion faces the supporting surface (91a,92a,93a) of the supporting portion (91,92,93); a provisionally tightening step of provisionally tightening a first fastening portion, in a state in which the first attachment surface of the attachment flange faces the second attachment surface of the opening portion, to couple the attachment flange to the opening portion; an adjusting step of adjusting a height of the installation surface relative to the supporting surface (91a,92a,93a) such that the axis line along which the pipe portion extends coincides with a horizontal plane, in the provisionally tightened state resulting from the provisionally tightening step; a coupling step of coupling the leg portion to the supporting portion (91,92,93) with a second fastening portion (70,80) after the adjusting step; a finally tightening step of finally tightening the first fastening portion after the coupling step, and an attachment step of attaching a spray nozzle to each of the plurality of nozzle holders, the spray nozzle configured to inject the liquid absorbent upward in the vertical direction. an insertion step of inserting the spray pipe from the opening portion; a positioning step of positioning the spray pipe such that a state is established in which the installation surface of the leg portion faces the supporting surface (91a,92a,93a) of the supporting portion (91,92,93); a provisionally tightening step of provisionally tightening a first fastening portion, in a state in which the first attachment surface of the attachment flange faces the second attachment surface of the opening portion, to couple the attachment flange to the opening portion; an adjusting step of adjusting a height of the installation surface relative to the supporting surface (91a,92a,93a) such that the axis line along which the pipe portion extends coincides with a horizontal plane, in the provisionally tightened state resulting from the provisionally tightening step; a coupling step of coupling the leg portion to the supporting portion (91,92,93) with a second fastening portion (70,80) after the adjusting step; a finally tightening step of finally tightening the first fastening portion after the coupling step, and an attachment step of attaching a spray nozzle to each of the plurality of nozzle holders, the spray nozzle configured to inject the liquid absorbent upward in the vertical direction.