A reaction column (12) to which a raw material mixture (11) containing a mono-lower-alkylamine (AA: raw material I) and an alkylene oxide (AO: raw material II) is supplied, an unreacted raw material distillation column (14) that separates an unreacted raw material (15) from a reaction product (13a) (containing the unreacted raw material (15), a target reaction product (monomer) (17), and a by-product (dimer) (18)), and a flash drum (16) to which a reaction product (13b) (containing the target reaction product (monomer) (17) and the by-product (dimer) (18)) is supplied, the flash drum (16) separating a mono-lower-alkyl monoalkanolamine (monomer, the target reaction product 17) in a gas state, are provided.