A cooling structure of an internal combustion engine includes: a cooling water introducing port provided on one end side of a cylinder block; and a water jacket provided so as to surround a cylinder bore wall, wherein cooling water is introduced from the cooling water introducing port into the water jacket, the cooling water is branched to flow to a portion on an intake side and a portion on an exhaust side of the water jacket of the cylinder block of the internal combustion engine, and the cooling water is supplied from a cylinder block side to a cylinder head side, the cooling structure of the internal combustion engine, further comprising a first regulation portion that regulates a flow of the cooling water supplied to the cylinder head side.