This ply separation strength tester is designed to be used together with a burst strength tester to measure ply separation strength of paperboard. The principle is as follows: the specimen, composed of two pieces of paperboard bonded together and cut off in the shape of a doughnut, as shown in the figure, is bonded in a state of being pinched between the circular plate and the hole-less metallic plate to be inserted into the burst strength tester for pressure application. During being pressurized, the rubber diaphragm inflates toward the hole-less disc. The gauge reads a maximum pressure at the moment ply separation occurs.