A rotary cutter for a mower is described including at least one cord of a predetermined length extending from the cutting head driven for rotation, and at least one fastening element for fixing one end of the cord to the cutting head main body. The fastening element includes a pair of ratchets. Each ratchet pivots for turning on the main body and is constantly applied with a rotational force in a predetermined direction by a spring. The cord is clamped and fastened by the ratchets. An inner end portion of the cord inserted from the sidewall of the main body is fastened by the fastening element, and the cord is removed and replaced with a new cord when the cord is worn out. The fastening element including a pair of ratchets with each ratchet of the pair including at least two or more sheets of laminated ratchets.