Yu AIDA-----------* 1Norihisa HASHIMOTO-ー * 2The natural frequencies and the mode shapes of deck plate slabs in steel structure buildings were measured. An eigenvalue analysis was carried out on the measured slab by the finite element method with coupling of in-plane and bending displacement, and the result showed good correspondence with the measurement. By using the eigenvalue analysis by the finite element method, conversion of deck plate slabs to flat plates were studied, and relation between ratio of side length in mode shape component and converted plate thickness was shown. The conversion method was verified in the vibration analysis of the actual building floor. The natural frequencies and the mode shapes of deck plate slabs in steel structure buildings were measured. An eigenvalue analysis was carried out on the measured slab by the finite element method with coupling of in-plane and bending displacement, and the result showed good correspondence with the measurement. By using the eigenvalue analysis by the finite element method, conversion of deck plate slabs to flat plates were studied, and relation between ratio of side length in mode shape component and converted plate thickness was shown. The conversion method was verified in the vibration analysis of the actual building floor.