A carbonitriding method that can improve the nitrogen permeating rate to render the carbonitriding process effective is directed to carbonitriding a workpiece formed of steel that contains at least 0.8 mass % of carbon by heating in an atmosphere including ammonia, carbon monoxide, carbon dioxide and hydrogen. The method includes an atmosphere control step, and a heating pattern control step. The atmosphere control step includes an undecomposed NH3 partial pressure control step, a CO/CO2 partial pressure control step, and an H2 partial pressure control step. The undecomposed NH3 partial pressure control step, the CO/CO2 partial pressure control step and the H2 partial pressure control step are carried out such that the partial pressure of hydrogen in the heat treatment furnace is at least 0.1 atmospheric pressure and not more than 0.3 atmospheric pressure, and γ defined by the following equation (1) is at least 2.0 and not more than 6.0, in the atmosphere control step. γ = a c * C NH 3 ( 1 ) where a c * = ( Pco ) 2 K × Pco 2 ( 2 ) PCO: partial pressure of carbon monoxide (atm); PCO2: partial pressure of carbon dioxide (atm), K: equilibrium constant at +CO22CO; CNH3: undecomposed ammonia concentration (volume %).