At the medical site, RTU (Ready-to-use, ready-to-use) preparations that require no complicated dissolution steps and can be administered immediately after dilution can be compared with freeze-dried preparations. The object of the present invention is to provide a solution formulation for pemetrexed containing pemetrexed, which suppresses the production of similar substances and suppresses the coloration of the solution during storage. According to the present invention, there is provided a method for producing a solution for injection, which is a method for producing a solution for injection containing pemetrexed disodium as an active ingredient. The method includes: (1) using pemetrexed disodium A step of preparing a solution for injection by using raw materials, and (2) a step of filling the above-mentioned solution for injection into a container for the preparation, and plugging the rubber portion with an atmosphere having an oxygen concentration of 1 vol% or less in the space, and The above raw materials contain a component whose relative retention time (RRT) is 0.41 ± 0.03 in a HPLC analysis of pemetrexed as an index in an HPLC analysis of 0.10% or less.於醫療現場中，要求相較於冷凍乾燥製劑，無需繁雜之溶解步驟且可稀釋後立即投予等方便性高之溶液形態之RTU(Ready-to-use，即用型)製劑。本發明之課題在於提供一種於保存期間抑制了類似物質之生成並且抑制了溶液著色之含有培美曲塞之注射用溶液製劑。根據本發明，提供一種注射用溶液製劑之製造方法，其係含有培美曲塞二鈉作為有效成分之注射用溶液製劑之製造方法，該方法包括：(1)使用含培美曲塞二鈉之原料而製備注射用溶液之步驟、及(2)將上述注射用溶液填充於製劑用容器中，以空隙部分之氧氣濃度為1體積%以下之氣氛藉由橡皮塞加以塞緊之步驟，且上述原料含有0.10%以下之HPLC分析中以培美曲塞之保持時間(RT)作為指標之相對保持時間(RRT)為0.41±0.03的成分。
