[Problem] To provide an internal combustion engine that can suitably realize suppression of a piston slap noise and reduction of friction. [Solution] An internal combustion engine is provided with: an engine body 2 that defines a cylinder bore 3; a piston 4 that is housed in the cylinder bore 3; and a crankshaft 8 that is coupled to the piston 4 via a connecting rod 6 and that is rotatably supported by the engine body 2. The piston has a skirt. The cylinder bore has: a first region A1 defined as a range, in a cylinder axial direction X, closer to a top dead center side than a first piston position; a second region A2 defined as a range, in the cylinder axial direction, closer to a bottom dead center side than a second piston position set closer to the bottom dead center than the first piston position; and a connection region A3 positioned between the first region and the second region. The diameter of the cylinder bore is smaller in the first region than in second region, and the connection region smoothly connects the first region and the second region.