Internal combustion engine

An internal combustion engine comprising a blow-by gas processing device that processes blow-by gas that has leaked from a combustion chamber to a crankcase, by causing the blow-by gas to return to an intake passage, wherein:

• the blow-by gas processing device includes an oil separator, a connection pipe, and a pressure sensor;
• the oil separator is provided in a cylinder head cover, includes a main chamber and a sub-chamber provided inside, and includes a joint portion constituting the sub-chamber and a throttle portion connecting the sub-chamber to the main chamber;
• the sub-chamber is a first sub-chamber and a second sub-chamber partitioned by a partition wall;
• a communication hole connecting the first sub-chamber and the second sub-chamber is provided in the partition wall;
• the first sub-chamber is connected to the main chamber via the throttle portion;
• a first connection port connected to the first sub-chamber and a second connection port connected to the second sub-chamber are provided in the joint portion;
• the first connection port is connected to the intake passage via the connection pipe;
• and
• the second connection port is connected to the pressure sensor.