A color thermosensitive recording sheet includes a support and at least yellow, magenta and cyan thermosensitive coloring layers formed thereon. A color thermal printer has conveyor rollers for moving the recording sheet relative to a thermal head. The thermal head initially heats the recording sheet for recording on the yellow coloring layer. A yellow fixing lamp applies near ultraviolet rays to the recording sheet to fix the yellow coloring layer optically. The thermal head heats the recording sheet for recording on the magenta coloring layer. A magenta fixing lamp applies ultraviolet rays to the recording sheet to fix the magenta coloring layer optically. An irradiance sensor detects irradiance of the magenta fixing lamp during the magenta fixation. A refixing check circuit obtains a minimal irradiance of the detected irradiance, compares the minimal irradiance with a limit irradiance, and outputs a refixing command if the minimal irradiance is equal to or smaller than the limit irradiance. A driver turns on the magenta fixing lamp in response to the refixing command. A conveying speed determiner determines a speed of the conveyor rollers for the refixation according to the minimal irradiance. The conveyor rollers are operated at the determined speed while the magenta fixing lamp is turned on. The magenta fixing lamp refixes the recording sheet optically. Finally the thermal head heats the recording sheet for recording on the cyan coloring layer.