In the case of acrylic acid or an acrylic ester, for example, this term refers to a solution of acrylic acid and an acrylic ester which contains by-products such as acetic acid, propionic acid, acrolein, maleic acid, water, and formalin which arise when the acrylic acid is obtained by the reaction of catalytic gas phase oxidation (JP-A-09-227,445 refers, for example).