A disposable diaper, comprising: a chassis including at least a front body and a back body; a top sheet arranged on at least part of the chassis, the top sheet being liquid permeable; a back sheet arranged on the top sheet at one side in a thickness direction of the top sheet, the back sheet being liquid impermeable; and an absorbent core arranged between the top sheet and the back sheet, the absorbent core being liquid retainable, wherein the front body and the back body are joined and secured together at a predetermined location in advance or upon wearing the disposable diaper to provide a pants shape with a trunk opening and a pair of leg openings, the front body is formed with one of an openable part or an opening, the openable part and the opening each being formed by connecting two points positioned at a distance from each other across a substantial center line extending in a longitudinal direction of the disposable diaper and bisecting a width direction thereof, the openable part allowing the front body to be separated at a predetermined site, and the opening allowing the front body to open at a predetermined site, wherein an opening stretch area is defined to connect a substantially central portion of the back body adjacent to the trunk opening and a predetermined location of the front body adjacent to at least one of the openable part or the opening along the outer perimeter of the pants shape, at least part of the opening stretch area being able to expand and contract, and wherein the opening stretch area includes an elastic opening area arranged at a predetermined location on the side of the openable part or the opening in a leg direction of the pants shape. a chassis including at least a front body and a back body; a top sheet arranged on at least part of the chassis, the top sheet being liquid permeable; a back sheet arranged on the top sheet at one side in a thickness direction of the top sheet, the back sheet being liquid impermeable; and an absorbent core arranged between the top sheet and the back sheet, the absorbent core being liquid retainable, wherein the front body and the back body are joined and secured together at a predetermined location in advance or upon wearing the disposable diaper to provide a pants shape with a trunk opening and a pair of leg openings, the front body is formed with one of an openable part or an opening, the openable part and the opening each being formed by connecting two points positioned at a distance from each other across a substantial center line extending in a longitudinal direction of the disposable diaper and bisecting a width direction thereof, the openable part allowing the front body to be separated at a predetermined site, and the opening allowing the front body to open at a predetermined site, wherein an opening stretch area is defined to connect a substantially central portion of the back body adjacent to the trunk opening and a predetermined location of the front body adjacent to at least one of the openable part or the opening along the outer perimeter of the pants shape, at least part of the opening stretch area being able to expand and contract, and wherein the opening stretch area includes an elastic opening area arranged at a predetermined location on the side of the openable part or the opening in a leg direction of the pants shape.