An opposed-piston internal combustion engine is configured so that one cylinder is provided with two pistons and these pistons reciprocate symmetrically with each other. The engine comprises: an expanded chamber formed at part of a wall of the cylinder positioned between the two pistons when a volume between the two pistons is the minimum and extending in a radial direction of the cylinder; an intake passage communicated with the expanded chamber; an exhaust passage communicated with the expanded chamber; an intake valve opening and closing the intake passage with respect to the expanded chamber; an exhaust valve opening and closing the exhaust passage with respect to the expanded chamber, and crankshafts respectively connected to the pistons. The expanded chamber formed so as to stick out from a wall surface of the cylinder in the same direction as axes of rotation of the crankshafts.