The present invention provides a production method of a thienopyrimidine derivative or a salt thereof which has a gonadotropin releasing hormone (GnRH) antagonistic action with high quality in high yield. The present invention provides a method of producing a thienopyrimidine derivative, which comprises reacting 6-(4-aminophenyl)-1-(2,6-difluorobenzyl)-5-dimethylaminomethyl-3-(6-methoxypyridazin-3-yl)thieno[2,3-d]pyrimidine-2,4(1H,3H)-dione or salt thereof, 1,1′-carbonyldiimidazole or a salt thereof and methoxyamine or a salt thereof, and the like.