Abstract: A four-rotor type rotary piston engine including an eccentric shaft assembly composed of three elements; a main eccentric shaft defining the rotational axis of each rotor and first and second auxiliary eccentric shafts coupled to opposite end of the main eccentric shaft. The auxiliary eccentric shafts and the main eccentric shaft art fitted by tapered surfaces and have a small clearance therebetween in the radial direction to enable smooth coupling. The main eccentric shaft has two rotor eccentric journals for the second and the third cylinders. At first, the second and the third cylinders are installed. After that, the auxiliary eccentric shafts having rotor eccentric journals are coupled with the main eccentric shaft, and finally the first and the fourth cylinders are installed.