An objective of the present invention is to improve the solubilityof 3 - [2-fluoro-5 -(2,3 -difluoro-6-methoxybenzyloxy)-4-methoxyphenyl]-2,4-dioxo-1,2,3,4-tetrahydrothieno[3,4-d]pyrimidine-5-carboxylic acid.The present invention provides3 - [2 -fluoro-5 -(2,3 -difluoro-6-methoxybenzyloxy)-4-methoxyphenyl] -2,4-dioxo-1,2,3,4-tetrahydrothieno[3,4-d]pyrimidine-5-carboxylicacid choline salt has excellentsolubility and storage stability.