The present invention relates to a manufacturing method of mechanical pulp from stalks of corn, an annual plant, by adding a digesting agent and, prior to, during or following addition of the digesting agent, completely separating fibrous material from the cornstalks by mechanical refining. The method of the present invention comprises: (1) a first digesting process of adding 0.5-5 parts by weight of a digesting agent selected from caustic soda, sodium carbonate, sodium sulfite, sodium bisulfite, sodium sulfide, sodium oxide and sodium sulfate, or a digesting agent selected from a mixture of more than one of each to 100 parts by weight of crushed cornstalks, and of separating and digesting the solid substance after impregnating of the mixture; (2) a second digesting process of adding 5-10 parts by weight of more digesting agent to the digested solid substance obtained from the first digesting process, and of separating and digesting the solid substance after impregnating of the mixture; and a refining process of separating the solid substance to be digested and refining it mechanically, which is performed prior to, during or following the second digesting process.