Disclosed herein is a clamping structure for mounting a reducer. The clamping structure include a clamp body 431 that is formed at opposite sides thereof with a door connecting portion 431a and a latch member connecting portion 431b to form a rectangular shape with one open side, and a clamp door 434 that can be rotated about a hinge pin 433 formed at the door connecting portion 431a, wherein the clamp door 431 is formed at one side thereof with a fastening portion, to which a latch portion of a latch member 450 is latched to allow a lever handle 446 of a lever member 440 to be pushed, and wherein the lever member 440 is coupled at one end thereof to the latch member connecting portion 450 of the clamp body 431 by a hinge 444 and is rotatably coupled at a middle thereof to the latch member 450 by a hinge 454 of the latch member 450.