In sequential steps including preparing a solution of a compound having —CF2O— as a linking group, bringing the solution into contact with a purifying agent or not bringing the solution into contact with a purifying agent, subsequently performing crystallization from the obtained solution for crystallization to precipitate a crystal, collecting the crystal by filtration, and drying the crystal, steps from the initiation of the crystallization step to the end of the drying step are performed in an atmosphere having an oxygen concentration of 3% by volume or less.