The strength of marine cylindrical boilers has been calculated by the Classification Societies rules, which are originated from C. Bach's experiments, but neither theoretical nor experimental certifications are made to confirm these rules. The authors tested two boilers, one of which is a used cylindrical boiler with dry combustion chamber landed because of the bulging of the rear end plate, and the other a boiler of the same type newly manufactured to replace it. The stresses of various places of these two boilers were measured by SR-4 gauges, and the rupture test by water pressure was also carried out on the landed boiler. Investigating the abovementioned tests, the followings are mainly concluded : 1) Shell plates are amply strong at where remoted from openings, which, however, can be the weakest point if they are poorly reinforced, carelessly cut or horizontally neighbored each other. 2) The values of the factor C ₁ given in the rules for various supports are adequate as. long as they are not in contact with flame. 3) The allowable stress of the supported flat plates is ascertained to be 17-18kg/mm². 4) Tube holes distributed on end plate give no remarkable influence to the plate strength. It is considered that the permissible supporting distance crossing the tube holes line may be corrected by multiplying the factor : ( p - d /2)/ p . 5) The current calculating method for the area supported by stay or stay tube gives smaller value when neighboring support is a flanging. Several points remained unclarified and succeeding test is planned.