A medical device packaging container (1, 1a-c) comprising: a tray having an upper end opening portion and a medical device storage portion; and a sheet-like lid portion that closes the upper end opening portion of the tray, wherein the tray includes an annular flange portion provided on an entire periphery of the upper end opening portion, the packaging container includes an annular heat seal portion that peelably fixes the sheet-like lid portion on the annular flange portion and has a rectangular outer edge, the annular heat seal portion includes a first-side seal portion and a second-side seal portion, which are opposed to each other, and a third-side seal portion and a fourth-side seal portion, which are positioned between the first-side seal portion and the second-side seal portion and opposed to each other, the sheet-like lid portion includes a flap portion protruding from the first-side seal portion of the annular heat seal portion, one end of the third-side seal portion and one end of the fourth-side seal portion are positioned on the second-side seal portion side by a predetermined length with respect to the first-side seal portion, the first-side seal portion includes a central seal portion, a first side-part seal portion extending obliquely and connecting one end of the central seal portion with the one end of the third-side seal portion, and a second side-part seal portion extending obliquely and connecting the other end of the central seal portion and the one end of the fourth-side seal portion, characterized in that the first-side seal portion includes two central seal extension portions (54, 54a, 55, 55a) extending from an outer edge of the central seal portion and separated from each other by a predetermined length, a first side-part seal extension portion extending from an outer edge of the first side-part seal portion, and a second side-part seal extension portion extending from an outer edge of the second side-part seal portion, an outer edge of a distal end portion of the central seal extension portion (54, 54a, 55, 55a), an outer edge of a distal end portion of the first side-part seal extension portion, and an outer edge of a distal end portion of the second side-part seal extension portion have a rounded shape without corners, and a distance between the one end and the other end of the central seal portion is 45/100 to 55/100 of a distance between an outer edge of the one end of the third-side seal portion and an outer edge of the one end of the fourth-side seal portion. wherein the tray includes an annular flange portion provided on an entire periphery of the upper end opening portion, the packaging container includes an annular heat seal portion that peelably fixes the sheet-like lid portion on the annular flange portion and has a rectangular outer edge, the annular heat seal portion includes a first-side seal portion and a second-side seal portion, which are opposed to each other, and a third-side seal portion and a fourth-side seal portion, which are positioned between the first-side seal portion and the second-side seal portion and opposed to each other, the sheet-like lid portion includes a flap portion protruding from the first-side seal portion of the annular heat seal portion, one end of the third-side seal portion and one end of the fourth-side seal portion are positioned on the second-side seal portion side by a predetermined length with respect to the first-side seal portion, the first-side seal portion includes a central seal portion, a first side-part seal portion extending obliquely and connecting one end of the central seal portion with the one end of the third-side seal portion, and a second side-part seal portion extending obliquely and connecting the other end of the central seal portion and the one end of the fourth-side seal portion, characterized in that the first-side seal portion includes two central seal extension portions (54, 54a, 55, 55a) extending from an outer edge of the central seal portion and separated from each other by a predetermined length, a first side-part seal extension portion extending from an outer edge of the first side-part seal portion, and a second side-part seal extension portion extending from an outer edge of the second side-part seal portion, an outer edge of a distal end portion of the central seal extension portion (54, 54a, 55, 55a), an outer edge of a distal end portion of the first side-part seal extension portion, and an outer edge of a distal end portion of the second side-part seal extension portion have a rounded shape without corners, and a distance between the one end and the other end of the central seal portion is 45/100 to 55/100 of a distance between an outer edge of the one end of the third-side seal portion and an outer edge of the one end of the fourth-side seal portion.