A rotary cutter for a mower for mowing grass and the like with a cord of a desired length extending in a radial direction of a cutting head from the cutting head driven for rotation through a drive shaft is described. The rotary cutter includes at least one fastening element for fixing one end of the cord to a cutting head main body. An inner end portion of the cord inserted from a sidewall of the head main body is fastened by the fastening element. The cord is removed and replaced with a new cord when the cord is worn out. The cord fastening element is disposed near a drive shaft coupling shaft tube portion in the cutting head main body. A cord outlet for removing the cord is provided to the drive shaft coupling shaft tube portion side. The inner end portion of the cord extends outside from the cord outlet and along a sidewall of the drive shaft coupling shaft tube portion of the cutting head main body.