A production process by which tertiary phosphine with an attached sterically bulky hydrocarbon group is produced in a high yield and with high purity on an industrial scale through simple and safe operations by allowing a dialkylphosphinous halide to react with a Grignard reagent in the presence of a copper compound in an amount corresponding to 0.1 to 5% by mol based on the dialkylphosphinous halide to produce tertiary phosphine represented by the following formula (3): ##STR00001## wherein R.sub.1 and R.sub.2 are each a tertiary hydrocarbon group of 4 to 13 carbon atoms, and R.sub.3 is an alkyl group, an alkenyl group, an aryl group or the like.