A method of manufacturing an FIBC, comprising the steps of; a) Cutting flexible sheet material in the form of a rectangular envelope for forming a sidewall of the FIBC b) Marking the envelope such that the envelope has an upper portion (UP1, UP2, UP3) and a lower portion (LP1, LP2, LP3) c) Folding the right and left side edges of the upper portion (UP1, UP2, UP3) such that the upper portion has • a first upper portion in the form of a rectangle having a predetermined height (a), • a second upper portion in the form of an isosceles trapezoid whose upper base is wider than its lower base and whose side edge is inclined with an angle of α1 with respect to the vertical axis (y), • a third upper portion in the form of an isosceles trapezoid whose upper base is wider than its lower base and whose side edge is inclined with an angle of β1 with respect to the vertical axis (y), d) Folding the right and left side edges of the lower portion (LP1, LP2, LP3) such that lower portion has • a first lower portion in the form of a rectangle having a predetermined height (f), • a second lower portion is in the form of an isosceles trapezoid whose lower base is wider than its upper base and whose side edge is inclined with an angle of α2 with respect to the vertical axis (y) • a third lower portion is in the form of an isosceles trapezoid whose lower base is wider than its upper base and whose side edge is inclined with an angle of β2 with respect to the vertical axis (y), e) Tacking the folded side edges of the lower portion (LP1, LP2, LP3) and the upper portion (UP1, UP2, UP3), characterized by f) Providing four sidewalls by following steps a) to e) and stitching side edges of four sidewalls to each other such that all sidewalls of the FIBC are formed wherein the six portions (UP1, UP2, UP3, LP1, LP2, LP3) forming the sidewall of the FIBC are stacked such that a sidewall has narrowed width in its middle parts and full width in the first and last portions along the vertical direction, and wherein the angles made by the side edges of the trapezoids (LP1, LP2, UP1, UP2) are in the following ranges; 7 ° < α 1 < 15 ° 9 ° < α 2 < 16 ° 2 ° < β 1 < 7 ° 2 ° < β 2 < 7 ° g) Stitching a bottom wall to the lower edges of the four sidewalls, h) Stitching a plurality of handles to the sidewalls of the FIBC. a) Cutting flexible sheet material in the form of a rectangular envelope for forming a sidewall of the FIBC b) Marking the envelope such that the envelope has an upper portion (UP1, UP2, UP3) and a lower portion (LP1, LP2, LP3) c) Folding the right and left side edges of the upper portion (UP1, UP2, UP3) such that the upper portion has • a first upper portion in the form of a rectangle having a predetermined height (a), • a second upper portion in the form of an isosceles trapezoid whose upper base is wider than its lower base and whose side edge is inclined with an angle of α1 with respect to the vertical axis (y), • a third upper portion in the form of an isosceles trapezoid whose upper base is wider than its lower base and whose side edge is inclined with an angle of β1 with respect to the vertical axis (y), • a first upper portion in the form of a rectangle having a predetermined height (a), • a second upper portion in the form of an isosceles trapezoid whose upper base is wider than its lower base and whose side edge is inclined with an angle of α1 with respect to the vertical axis (y), • a third upper portion in the form of an isosceles trapezoid whose upper base is wider than its lower base and whose side edge is inclined with an angle of β1 with respect to the vertical axis (y), d) Folding the right and left side edges of the lower portion (LP1, LP2, LP3) such that lower portion has • a first lower portion in the form of a rectangle having a predetermined height (f), • a second lower portion is in the form of an isosceles trapezoid whose lower base is wider than its upper base and whose side edge is inclined with an angle of α2 with respect to the vertical axis (y) • a third lower portion is in the form of an isosceles trapezoid whose lower base is wider than its upper base and whose side edge is inclined with an angle of β2 with respect to the vertical axis (y), • a first lower portion in the form of a rectangle having a predetermined height (f), • a second lower portion is in the form of an isosceles trapezoid whose lower base is wider than its upper base and whose side edge is inclined with an angle of α2 with respect to the vertical axis (y) • a third lower portion is in the form of an isosceles trapezoid whose lower base is wider than its upper base and whose side edge is inclined with an angle of β2 with respect to the vertical axis (y), e) Tacking the folded side edges of the lower portion (LP1, LP2, LP3) and the upper portion (UP1, UP2, UP3), characterized by f) Providing four sidewalls by following steps a) to e) and stitching side edges of four sidewalls to each other such that all sidewalls of the FIBC are formed wherein the six portions (UP1, UP2, UP3, LP1, LP2, LP3) forming the sidewall of the FIBC are stacked such that a sidewall has narrowed width in its middle parts and full width in the first and last portions along the vertical direction, and wherein the angles made by the side edges of the trapezoids (LP1, LP2, UP1, UP2) are in the following ranges; 7 ° < α 1 < 15 ° 9 ° < α 2 < 16 ° 2 ° < β 1 < 7 ° 2 ° < β 2 < 7 ° g) Stitching a bottom wall to the lower edges of the four sidewalls, h) Stitching a plurality of handles to the sidewalls of the FIBC.