A method of filling a paste-type dental material into a cartridge, the cartridge including: a cartridge body made of a resin and including a cylindrical portion having two ends and an opening portion at one of the two ends and a nozzle portion provided at the other of the two ends of the cylindrical portion, and a piston made of a resin and disposed wholly inside the cylindrical portion to push out a content contained in the cylindrical portion of the cartridge body through the nozzle portion, wherein a pair of piston-side tapered surfaces or piston-side curved surfaces are formed between an annular peripheral wall surface of the piston and a pair of circular end surfaces of the piston located on both sides in a thickness direction of the piston, the pair of piston-side tapered surfaces or piston-side curved surfaces becoming smaller in radial dimension from the annular peripheral wall surface toward the pair of circular end surfaces, wherein the method comprises: filling the paste-type dental material from the opening portion of the cartridge into the cylindrical portion by moving the opening portion of the cylindrical portion of the cartridge along a kneading surface, on which the paste-type dental material is kneaded, with the opening portion being inclined; placing the piston on the kneading surface with the circular end surface of the piston contacting the kneading surface; and inserting the piston into the cylindrical portion by moving the opening portion of the cylindrical portion of the cartridge closer to the piston located on the kneading surface such that the piston-side tapered surface or the piston-side curved surface of the piston is fitted into the opening portion, and thereafter moving the one of the two ends of the cylindrical portion toward the kneading surface. a cartridge body made of a resin and including a cylindrical portion having two ends and an opening portion at one of the two ends and a nozzle portion provided at the other of the two ends of the cylindrical portion, and a piston made of a resin and disposed wholly inside the cylindrical portion to push out a content contained in the cylindrical portion of the cartridge body through the nozzle portion, wherein a pair of piston-side tapered surfaces or piston-side curved surfaces are formed between an annular peripheral wall surface of the piston and a pair of circular end surfaces of the piston located on both sides in a thickness direction of the piston, the pair of piston-side tapered surfaces or piston-side curved surfaces becoming smaller in radial dimension from the annular peripheral wall surface toward the pair of circular end surfaces, filling the paste-type dental material from the opening portion of the cartridge into the cylindrical portion by moving the opening portion of the cylindrical portion of the cartridge along a kneading surface, on which the paste-type dental material is kneaded, with the opening portion being inclined; placing the piston on the kneading surface with the circular end surface of the piston contacting the kneading surface; and inserting the piston into the cylindrical portion by moving the opening portion of the cylindrical portion of the cartridge closer to the piston located on the kneading surface such that the piston-side tapered surface or the piston-side curved surface of the piston is fitted into the opening portion, and thereafter moving the one of the two ends of the cylindrical portion toward the kneading surface.