Provided is an irinotecan formulation capable of supporting irinotecan and/or a salt thereof in a closed vesicle carrier at a high concentration and existing in blood for a long period of time by dramatically improved retentivity in blood compared to a conventionally known irinotecan liposome formulation. That is, an irinotecan formulation including a closed vesicle formed by a lipid membrane, in which irinotecan and/or a salt thereof is encapsulated at a concentration of at least 0.07 mol/mol (drug mol/membrane total lipid mol). There is an ion gradient between an inner aqueous phase and an outer aqueous phase in the irinotecan formulation. The closed vesicle is preferably liposome, in which only the outer surface of the liposome is preferably modified with a surface-modifying agent containing a hydrophilic polymer.