Abstract: In order to manufacture a pipe having a plurality of flow channels, by minimizing die jigs and steps, and thus to permit facilitation of manufacture, reduction of cost, and improvement of accuracy, the method of manufacturing a pipe comprises the steps of holding a sheet material at a center portion in the width direction thereof from both surfaces thereof by core members composing a split structure, folding both sides in the width direction of the sheet material in opposite directions and causing the thus folded both sides of the sheet material to follow the shapes of the outer surfaces of the core members, thereby forming a substantially an S-shaped cross-section.