There is provided a biocatalyst solvent which is capable of dissolving a biocatalyst in a liquid state while maintaining the activity of the biocatalyst at a high concentration from a low temperature to a high temperature, and is capable of storing the biocatalyst for a long time period; and a biocatalyst solution using the biocatalyst solvent. Disclosed is a biocatalyst solvent which consists of an ionic liquid including a quaternary ammonium cation represented by the following Formula (I) and an anion: [Chemical Formula 1] N+[Ra]n[Rb]4-n  (I) wherein Ra's each independently represent a hydroxyalkyl group having one or more hydroxyl groups and an a linear or branched alkyl moiety having 1 to 10 carbon atoms, in which the alkyl moiety may contain an oxygen atom; a carboxyalkyl group having one or more carboxyl groups and a linear or branched alkyl moiety having 1 to 10 carbon atoms, in which the alkyl moiety may contain an oxygen atom; or a hydroxycarboxyalkyl group having one or more hydroxyl groups, one or more carboxyl groups and a linear or branched alkyl moiety having 1 to 10 carbon atoms, in which the alkyl moiety may contain an oxygen atom; Rb's each independently represent a hydrogen atom, or a linear or branched alkyl group having 1 to 5 carbon atoms; and n represents an integer from 1 to 4.