An internal combustion engine includes a blow-by gas processing device. A cylinder head cover of the internal combustion engine is provided with a joint portion including a connection port to which a connection pipe is connected and a throttle portion having a passage sectional area smaller than a passage sectional area of the connection port. The joint portion is communicated with an inside of a cylinder head and a space in the joint portion serves as an oil separator. A pressure sensor is connected to the space in the oil separator via a connection passage. The connection passage includes a first pipe, a second pipe, and a chamber including a first connection port to which the first pipe is connected and a second connection port to which the second pipe is connected. The first connection port and the second connection port are open toward the same direction in the chamber.