An internal combustion engine comprising a blow-by gas processing circuit, a carburetor having an intake passageway extending laterally, the carburetor being connected to one side surface of the internal combustion engine, and an air cleaner placed above the carburetor and connected to an upstream end of the carburetor, wherein the blow-by gas processing circuit comprises: a blow-by gas lead-out pipe rising upward from the internal combustion engine; an upper pipe bending laterally from an upper end of the blow-by gas lead-out pipe; a descending pipe bending downward from an extremity portion of the upper pipe; a branch pipe branching off downward from an intermediate portion of the upper pipe, and connected to a blow-by gas outlet port which is opened to an intake passage; and a control valve placed in the branch pipe, and configured to open in accordance with a rise in boost negative pressure of the internal combustion engine; characterised by: the air cleaner having a cleaner element inside a cleaner case, the cleaner element being configured to partition the cleaner case into a pre-purification chamber and a purification completion chamber communicating with an air outlet pipe; the descending pipe being connected to a second joint pipe of the air outlet pipe; the blow-by gas processing circuit being arranged adjacent to and along one side surface of the air cleaner, with the blow-by gas lead-out pipe extending along the one side surface of the air cleaner, the upper pipe extending along the one side surface of the air cleaner, and the descending pipe extending along the one side surface so as to reach the purification completion chamber of the air cleaner; an insulator member interposed between the carburetor and the one side surface of the internal combustion engine, the intake passage penetrating the insulator member, and the control valve being placed in an upper portion of the branch pipe. a blow-by gas lead-out pipe rising upward from the internal combustion engine; an upper pipe bending laterally from an upper end of the blow-by gas lead-out pipe; a descending pipe bending downward from an extremity portion of the upper pipe; a branch pipe branching off downward from an intermediate portion of the upper pipe, and connected to a blow-by gas outlet port which is opened to an intake passage; and a control valve placed in the branch pipe, and configured to open in accordance with a rise in boost negative pressure of the internal combustion engine; the air cleaner having a cleaner element inside a cleaner case, the cleaner element being configured to partition the cleaner case into a pre-purification chamber and a purification completion chamber communicating with an air outlet pipe; the descending pipe being connected to a second joint pipe of the air outlet pipe; the blow-by gas processing circuit being arranged adjacent to and along one side surface of the air cleaner, with the blow-by gas lead-out pipe extending along the one side surface of the air cleaner, the upper pipe extending along the one side surface of the air cleaner, and the descending pipe extending along the one side surface so as to reach the purification completion chamber of the air cleaner; an insulator member interposed between the carburetor and the one side surface of the internal combustion engine, the intake passage penetrating the insulator member, and the control valve being placed in an upper portion of the branch pipe.