The present invention provides a process that enables a substituted methylamine compound which is useful as an intermediate for the production of agricultural chemicals and medicines, to be produced easily, with good yield, and at low cost, and also provides a production intermediate thereof. The process comprises a step of reacting a hexamethylenetetraammonium salt compound represented by a formula (I) with a base to obtain an N-methylidene-substituted methylamine oligomer represented by a formula (II) or a mixture of two or more of the oligomers, and a step of hydrolyzing the N-methylidene-substituted methylamine oligomer represented by formula (II) or the mixture of two or more of the oligomers in the presence of an acid. (wherein A represents an organic group that is either a hydrocarbon group or a heterocyclic group, or said organic group that has a substituent, R represents a hydrogen atom, an organic group that is either a hydrocarbon group or a heterocyclic group, or said organic group that has a substituent, L represents a halogen atom and the like, and n represents an integer of 2 to 20)