PURPOSE: To simplify the construction of an assembly in which an engine cylinder and an intake pump cylinder are arranged in tandem and pistons in the cylinders are fixed to a common piston rod, by providing a scavenging means and an intake means at the rod hole of each of the cylinders.

CONSTITUTION: An engine cylinder 1 and an intake pump cylinder 20 are coaxially coupled to each other. An engine piston 3 and an intake piston 21 are fitted in the cylinders 1, 20 and fixed to a common piston rod 5. A pair of a scavenging valve hole 25 and an intake valve hole 26 are provided at the midway portion of each of rod holes provided in a pair of cylinder heads 2 for the cylinder 1 and a pair of covers 22 for the cylinder 20. The valve holes 25, 26 are opened and closed by a scavenging spool valve 27 and an intake spool valve 28 which are provided as grooves in the piston rod 5. The scavenging valve hole 25 near the cylinder 20 and the intake valve hole 26 near the other cylinder 1 are connected to each other through an intake pipe 29. The other scavenging valve hole 25 and the other intake valve hole 26 are also connected to each other through another intake pipe 29.