A disposable diaper production method for producing a disposable diaper having a main body portion that extends from a front belly part to a back part via a crotch part and a pair of side portions that covers side surfaces of a lumbar part of a wearer when the disposable diaper is worn, the method comprising: a constituents producing step of producing constituents formed of the main body portion and four side pieces (3, 4) each of which protrudes from both ends of the main body portion in a longitudinal direction to both outer sides in a width direction of the main body portion; a folding step of folding the main body portion in the longitudinal direction to form two sets of two mutually-overlapping side pieces (3, 4); a joining step of joining the mutually-overlapping side pieces (3, 4) to each other for each set of the two side pieces (3, 4) of the folded constituents to form the pair of side portions; a cutting step of rotating and driving a driving shaft to rotate a pair of driving blades about an axial line of the driving shaft and compressing each set of the two side pieces (3, 4) joined to each other between the respective driving blades and a pair of facing blades facing the respective driving blades to cut off end portions of the two side pieces (3, 4) uniformly; and a conveying step of conveying the constituents in the longitudinal direction in a state in which both side portions in the width direction of a cutting position of the respective side pieces (3, 4) are held for each set of the two side pieces (3, 4) when the respective side pieces (3, 4) are cut off in the cutting step, wherein the cutting step rotates the driving blades at a rate faster than a conveying rate of the constituents in the conveying step, wherein a plurality of support rollers (29, 30) is provided on both sides of the pair of driving blades in the width direction at a periphery of the driving shaft so as to be rotatable about the axial line of the driving shaft with respect to the driving shaft, and wherein the conveying step circulates a plurality of belts (34, 35, 36, 37, 38) that sandwiches the constituents from both front and rear sides thereof on both the side portions in the width direction of the cutting positions along a prescribed path in a state of being pressed to peripheral surfaces of the respective support rollers (29, 30). a constituents producing step of producing constituents formed of the main body portion and four side pieces (3, 4) each of which protrudes from both ends of the main body portion in a longitudinal direction to both outer sides in a width direction of the main body portion; a folding step of folding the main body portion in the longitudinal direction to form two sets of two mutually-overlapping side pieces (3, 4); a joining step of joining the mutually-overlapping side pieces (3, 4) to each other for each set of the two side pieces (3, 4) of the folded constituents to form the pair of side portions; a cutting step of rotating and driving a driving shaft to rotate a pair of driving blades about an axial line of the driving shaft and compressing each set of the two side pieces (3, 4) joined to each other between the respective driving blades and a pair of facing blades facing the respective driving blades to cut off end portions of the two side pieces (3, 4) uniformly; and a conveying step of conveying the constituents in the longitudinal direction in a state in which both side portions in the width direction of a cutting position of the respective side pieces (3, 4) are held for each set of the two side pieces (3, 4) when the respective side pieces (3, 4) are cut off in the cutting step, wherein the cutting step rotates the driving blades at a rate faster than a conveying rate of the constituents in the conveying step, wherein a plurality of support rollers (29, 30) is provided on both sides of the pair of driving blades in the width direction at a periphery of the driving shaft so as to be rotatable about the axial line of the driving shaft with respect to the driving shaft, and wherein the conveying step circulates a plurality of belts (34, 35, 36, 37, 38) that sandwiches the constituents from both front and rear sides thereof on both the side portions in the width direction of the cutting positions along a prescribed path in a state of being pressed to peripheral surfaces of the respective support rollers (29, 30).