The present invention provides a process for producing (E)-N-monoalkyl-3-oxo-3-(2-thienyl)propenamine represented by Formula (1); wherein R is a C1-4 alkyl, the method comprising the steps of: maintaining a solution containing (Z)—N-monoalkyl-3-oxo-3-(2-thienyl)propenamine dissolved therein at 25° C. or below to deposit crystals and separating crystals having a particle diameter of 100 μm or less from the deposited crystals; and a process for producing (E,Z)—N-monoalkyl-3-oxo-3-(2-thienyl)propenamine comprising the steps of: reacting an alkali metal salt of β-oxo-β-(2-thienyl)propanal with a monoalkylamine compound; adding a water-insoluble organic solvent to the resulting reaction mixture; adding seed crystals containing (E)-N-monoalkyl-3-oxo-3-(2-thienyl)propenamine to an organic layer obtained by conducting separation; and keeping the resulting mixture at 25° C. or below.