To effectively suppress strong knock that occurs in the operating region of high load and high rotation in a specific engine having a pre-chamber in a combustion chamber, the engine includes a piston that defines a combustion chamber together with a cylinder block and a cylinder head. The combustion chamber includes a sub-chamber and a main chamber separated from the sub-chamber by a pre-chamber. The specific ratio obtained by dividing a bore/stroke ratio of a cylinder by the volume of the sub-chamber is greater than or equal to 6.6 cm−3 and less than or equal to 57.6 cm−3.