To simplify the configuration of a pump mechanism and a pump having an opposed piston type internal combustion engine for driving the same.

An internal combustion engine comprises two outer pistons 105, 107, and a central piston 109 disposed therebetween. The 3 pistons are interlocked with elements of a pump mechanism with interlocking members 182a, 182b, 182c so that the elements of the pump mechanism are driven according to movement of the piston. The interlocking members elongate to the elements of the pump mechanism through a slot provided in the cylinder along a standing shaft of a cylinder.