An internal combustion engine is provided with a sub chamber which is communicated with a combustion chamber and with a variable volume device which changes the volume of the sub chamber. The variable volume device includes a communication part arranged at a cylinder head and formed into a tubular shape, a movement member which is formed in a tubular shape so as to engage with the inside of the communication part and which has a closed end at the side facing the combustion chamber, and a support part which has a projecting part which engages with the inside of the movement member. The variable volume device has the space at the inside of the communication part divided by a movement member and is formed with a sub chamber and gas chamber. The movement member has an open end at the opposite side to the side facing the combustion chamber and is formed so that gas in the gas chamber which leaks from the parts where the movement member and the projecting part contact is discharged to the outside of the cylinder head.