To reduce the processing time for in-line carburizing, we performed ultra-rapid carburizing and quenching using induction heating. Conventional gas carburizing and vacuum carburizing（low-pressure carburizing）are treated at 1203～1323 K, which is lower than the eutectic temperature. In contrast, in this study, the carburizing temperature of 1523 K was higher than the eutectic temperature for speeding up this process. The SCM420 sample was rapidly heated to the treatment temperature by induction heating. CH₄ and N₂ were mixed and treated at atmospheric pressure at 5 vol％ CH₄ and 10 vol％ CH₄. For ultra-rapid carburizing, the total carburizing depth was proportional to the square root of the carburizing time, following the parabolic law, regardless of the CH₄ concentration. We obtained that the amount of carbon penetrating the steel from the atmosphere was proportional to the carburizing time. In other words, the carbon penetration rate was approximately constant during the carburizing process. The processing time for the ultra-rapid carburizing and quenching at 1523 K in this study was only 5％ of that for the conventional gas carburizing at 1203 K.