The present invention relates to a grass cutter having a gear auxiliary device and a safety plate, which has been developed to reduce damage to a safety plate (formed to prevent damage from a blade for cutting grass colliding with objects on the ground) and to a support (for supporting the safety plate). The grass cutter has a gear auxiliary device and a safety plate that is fixed below a blade mounted on a rotating shaft that projects from and rotates at the bottom of a handle shaft of the grass cutter. After the rotating shaft of the grass cutter has the blade mounted thereon, a fixer coupling nut (having a screw thread formed on an inner peripheral surface thereof to correspond to a screw thread formed on the outer peripheral surface of the rotating shaft, and a screw thread formed on an outer peripheral surface thereof) is mounted on the rotating shaft, and then a second nut is coupled to fix the blade. The grass cutter further comprises: a rotator and a fixer. The rotator has a screw thread formed on the upper inner peripheral surface thereof to couple with the outer peripheral surface of the fixer coupling nut, has an inner peripheral surface therebelow that is reduced in stages by means of first and second stepped portions, and has a ring-shaped fine guide groove formed in the undersurface thereof. The fixer has a guide rib covering a part of the bottom of the outer diameter surface of the rotator and a guide projection correspondingly inserted in the guide groove formed on a top surface of the fixer, has a third stepped portion formed thereon having an outer diameter that is reduced to fit closely against the bottom inner diameter of the rotator and having an inner diameter at a region corresponding to the second stepped portion that is reduced by a step, and has a fixer shaft projecting from the center of the bottom surface thereof and having a screw thread formed on the outer peripheral surface of the bottom. The gear auxiliary device further has a first b
