ENGINE

The present invention provides an engine in which the positions of each valve and a spark plug are optimized with respect to an extension space. The engine 10 comprises: a cylinder chamber 12, an intake valve 18, an exhaust valve 19, and a spark plug 26. Inside the cylinder chamber 12, opposing pistons are arranged so as to reciprocate. An extension space 33 is a space continuous from an intermediate portion of the cylinder chamber 12 toward a side. On a first side surface 3313 of the extension space 33, a first intake valve 181 is arranged. On a second side surface 3314 of the extension space 33, which opposes the first side surface 3313, a first exhaust valve 191 and a first spark plug 261 are arranged.