Provided is an operation machine wherein a liquid reductant tank having a sufficient capacity can be easily installed. The base end portion of a boom of the operation machine is rotatably supported by a boom foot pin (12) extending between brackets (11) on both sides of an upper turn body (2) so as to be horizontally inserted or removed. A storage box installation portion (18) is provided at a location surrounded by the bracket (11) on one side, a side plate portion of a fuel tank (6), and a turn body frame portion (17) provided on the peripheral edge portion of the upper turn body (2). A storage box (5) is installed so as to sterically cover the storage box installation portion (18). A urea solution tank (21) made of resin, storing urea solution for reducing nitrogen oxide is installed at the rearmost position adjacent to the fuel tank (6), on the inside of the storage box (5). A pail (24), a refueling pump (25), a hose (26), and a tool box (27) are stored in front of the urea solution tank (21).