side door hinge mechanism in a motor vehicle, comprising: a pair of top rotary shafts (22A, 24A) on the side of a side door (12), supported on an end portion (14) of the side door (12) at the top portion thereof; a pair of top rotary shafts (26A, 28A) on the side of a vehicle body, supported on the vehicle body; a pair of two bottom rotary shafts (22B, 24B, 26B, 28B) on the sides of the side door (12) and the vehicle body, aligned with the four top rotary shafts (22A, 24A, 26A, 28A); a top control arm (30A) rotatably connected at opposite ends thereof to one (26A) of the top rotary shafts (26A, 28A) on the side of the vehicle body and one (22A) of the top rotary shafts (22A, 24A) on the side of the side door (12); a bottom control arm (30B) rotatably connected at opposite ends thereof to the bottom rotary shafts (26B, 22B) aligned with the top rotary center shafts (26A, 22A) of the top control arm (30A); and a main arm (32) formed integrally in the vertical direction and rotatably connected at end portions thereof in the vertical and the lateral directions to the remaining top and bottom rotary shafts (24A, 24B, 28A, 28B), the main arm (32) including a large diameter pipe portion (33) elongated in the vertical direction, supported at the top end and bottom thereof by the top and bottom rotary shafts (28A, 28B) on the side of the vehicle body, and a top arm (38A) and a bottom arm (38B) each being of a generally triangular shape tapered toward the forward end thereof, the proximal ends of which are connected to the upper and lower end portions of the pipe portion (33), and the forward ends of which are connected to the top and bottom rotary shafts (24A, 24B) on the side of the side door (12).