To provide an intake structure of internal combustion engine capable of heating a carburetor at a low cost with simple structure not needing a space.

This intake structure of internal combustion engine 10 comprises the carburetor 20 mounted on intake passages (an intake manifold 18 and an intake pipe 19) halfway via metallic mounting plates 21, 22 sandwiching the front. and the rear of the carburetor 20. A hot water passage lying approximately at right angles to the intake passage direction is formed on the mounting plate 21 to let hot water to flow in the hot water passage Since the hot water passage is formed on the existing mounting plate 21 in order to mount the carburetor 20 thereon, the carburetor can be heated at a low cost with simple structure not needing space for preventing freezing of the carburetor 20 to increase smoothly rotation of the engine 10.