Disclosed is a comb-vibration preventing structure for a hair cutter including a cutter body and a cutting-length adjusting comb adapted to be fitted on an upper portion of the cutter body and vertically moved in a sliding manner so as to adjust a cutting length. The comb-vibration preventing structure comprises a wire spring for urging each of opposite inner side surfaces of the cutting-length adjusting comb in an outward thrusting manner in an opposite lateral direction. The comb-vibration preventing structure can prevent vibrations of the cutting-length adjusting comb to reduce chatter noises, and achieve enhanced usability while facilitating quality control.