A construction machine in which a fuel tank is mounted to enable the machine to work for a long time through one fueling. The construction machine includes a lower driving structure; an upper frame mounted on the lower driving structure to horizontally swivel; a cab mounted on one side of an upper surface of the upper frame; a mounting portion formed to extend from a rearmost side of a center frame of the upper frame; a counterweight detachably installed on the mounting portion through a fastening member, and having a storage space portion of which a front surface and an upper surface are opened; a fuel tank coupled to the storage space portion and detachably fixed to the mounting portion through another fastening member that fixes the counterweight to the mounting portion in order to separate the fuel tank from the counterweight in a state where the counterweight that is coupled to the fuel tank is mounted on the mounting portion; an engine room mounted in front of the fuel tank of the upper frame; and a hydraulic fluid tank installed on one side of a front portion of the engine room.