A resin part (10; 50; 60) that is configured such that its length is large relative to its width, wherein the resin part (10; 50; 60) has an asymmetrical shape in a thickness direction thereof, so that a portion in which an increase in internal temperature by heating is relatively quick is positioned closer to one end of the resin part (10; 50; 60) in the thickness direction while a portion in which an increase in internal temperature by heating is relatively slow is positioned closer to the other end of the resin part (10; 50; 60) in the thickness direction, and wherein the resin part (10; 50; 60) has an asymmetrical shape in a width direction thereof, so that the portion in which an increase in internal temperature by heating is relatively quick is positioned closer to one end of the resin part in the width direction while the portion in which an increase in internal temperature by heating is relatively slow is positioned closer to the other end of the resin part (10; 50; 60) in the width direction, characterized in that the portion in which an increase in internal temperature by heating is relatively quick is formed as a thinner portion whereas the portion in which an increase in internal temperature by heating is relatively slow is formed as a thicker portion, the resin part (10; 50; 60) is formed in such a way that stresses remain in the resin part (10; 50; 60) in a longitudinal contraction direction, and the resin part (10; 50; 60) is configured such that the thinner portion is quickly contracted in a longitudinal direction of the resin part (10; 50; 60) relative to the thicker portion when the thinner portion is quickly increased in internal temperature relative to the thicker portion in a process in which the resin part (10; 50; 60) is heated to a softening temperature thereof, so that the resin part (10; 50; 60) is helically deformed. wherein the resin part (10; 50; 60) has an asymmetrical shape in a thickness direction thereof, so that a portion in which an increase in internal temperature by heating is relatively quick is positioned closer to one end of the resin part (10; 50; 60) in the thickness direction while a portion in which an increase in internal temperature by heating is relatively slow is positioned closer to the other end of the resin part (10; 50; 60) in the thickness direction, and wherein the resin part (10; 50; 60) has an asymmetrical shape in a width direction thereof, so that the portion in which an increase in internal temperature by heating is relatively quick is positioned closer to one end of the resin part in the width direction while the portion in which an increase in internal temperature by heating is relatively slow is positioned closer to the other end of the resin part (10; 50; 60) in the width direction, characterized in that the portion in which an increase in internal temperature by heating is relatively quick is formed as a thinner portion whereas the portion in which an increase in internal temperature by heating is relatively slow is formed as a thicker portion, the resin part (10; 50; 60) is formed in such a way that stresses remain in the resin part (10; 50; 60) in a longitudinal contraction direction, and the resin part (10; 50; 60) is configured such that the thinner portion is quickly contracted in a longitudinal direction of the resin part (10; 50; 60) relative to the thicker portion when the thinner portion is quickly increased in internal temperature relative to the thicker portion in a process in which the resin part (10; 50; 60) is heated to a softening temperature thereof, so that the resin part (10; 50; 60) is helically deformed.