A pin assembly comprising: a resin pin (61; 61E; 61F; 61G; 61H) which is an injection molded piece and which comprises a round shaft portion, the round shaft portion being inserted into a first insertion hole and a second insertion hole in a first direction to connect a first plate and a second plate with each other in a second direction perpendicular to the first direction, the round shaft portion being sheared to cancel connection between the first plate and the second plate upon secondary collision of a vehicle, the first insertion hole and the second insertion hole being respectively provided in the first plate and the second plate opposing each other in the first direction; and a collar which comprises an inner circumference fitted to an outer circumference of the round shaft portion and an end face received by a reception surface of the second plate, which has higher hardness than the resin pin (61; 61E; 61F; 61G; 61H), characterized in that the collar is received in the first insertion hole, and which shears the round shaft portion in a shear plane along the end face upon the secondary collision, wherein the round shaft portion comprises at least one hollow hole (81; 81E; 81F; 81G; 81H) which is provided around an axis of the round shaft portion as a center, and which extends in an axial direction of the round shaft portion, and that an inner circumference of the hollow hole comprises: a straight portion (82; 82E; 82F; 82G) which has a generating line parallel to the axial direction of the round shaft portion and which is traversed by a plane comprising the end face of the collar; and a tapered portion (85; 85E; 85F; 85G) which is provided for release from a mold and which is connected to the straight portion (82; 82E; 82F; 82G) through a boundary portion disposed at a position separated from the end face of the collar in the axial direction of the round shaft portion. a resin pin (61; 61E; 61F; 61G; 61H) which is an injection molded piece and which comprises a round shaft portion, the round shaft portion being inserted into a first insertion hole and a second insertion hole in a first direction to connect a first plate and a second plate with each other in a second direction perpendicular to the first direction, the round shaft portion being sheared to cancel connection between the first plate and the second plate upon secondary collision of a vehicle, the first insertion hole and the second insertion hole being respectively provided in the first plate and the second plate opposing each other in the first direction; and a collar which comprises an inner circumference fitted to an outer circumference of the round shaft portion and an end face received by a reception surface of the second plate, which has higher hardness than the resin pin (61; 61E; 61F; 61G; 61H), characterized in that the collar is received in the first insertion hole, and which shears the round shaft portion in a shear plane along the end face upon the secondary collision, wherein the round shaft portion comprises at least one hollow hole (81; 81E; 81F; 81G; 81H) which is provided around an axis of the round shaft portion as a center, and which extends in an axial direction of the round shaft portion, and that an inner circumference of the hollow hole comprises: a straight portion (82; 82E; 82F; 82G) which has a generating line parallel to the axial direction of the round shaft portion and which is traversed by a plane comprising the end face of the collar; and a tapered portion (85; 85E; 85F; 85G) which is provided for release from a mold and which is connected to the straight portion (82; 82E; 82F; 82G) through a boundary portion disposed at a position separated from the end face of the collar in the axial direction of the round shaft portion. a straight portion (82; 82E; 82F; 82G) which has a generating line parallel to the axial direction of the round shaft portion and which is traversed by a plane comprising the end face of the collar; and a tapered portion (85; 85E; 85F; 85G) which is provided for release from a mold and which is connected to the straight portion (82; 82E; 82F; 82G) through a boundary portion disposed at a position separated from the end face of the collar in the axial direction of the round shaft portion.