The present invention provides acetone complexes of sulfoxide compounds, useful as medicines such as inhibitors of gastric acid secretion and anti-ulcer agents or as intermediates for production of medicines, a process for producing the same and a purification method of using the same. Namely, it is the acetone complexes of sulfoxide compounds or of pharmaceutically acceptable salts thereof represented by the following formula: (wherein R1 represents a hydrogen atom, a methoxy group or a difluoromethoxy group, R2 represents a methyl group or a methoxy group, R3 represents a 3-methoxypropoxy group, a methoxy group or a 2,2,2-trifluoroethoxy group, R4represents a hydrogen atom or a methyl group, n and m independently represent an integer of 1 to 4, and B represents a hydrogen atom, an alkali metal atom or ½ alkaline earth metal atom), which are obtained by treating the sulfoxide compounds or pharmaceutically acceptable salts thereof with acetone.