The invention provides a salt of (R)-3-[2-(2-hydroxymethylpyrrolidin-1-yl)ethyl]-5-methyl-2H-isoquinolin-1-one and a crystal thereof. (R)-3-[2-(2-Hydroxymethylpyrrolidin-1-yl)ethyl]-5-methyl-2H-isoquinolin-1-one monophosphate is a compound which is chemically stable, has high solubility, and shows less weight change due to humidity as compared to a free form and monohydrochloride dihydrate, and is superior as a bulk drug for pharmaceutical products.