In this paper the author investigated the causes of the damages of the Mutsu Joint Office which was slightly damaged by the 1968 Tokachi-oki Earthquake analyzing the elastic dynamic behavior, the Earthquak response, and the strength of the members. This building consists of reinforced concrete and has 3 storeys and 1 storey basement. The building has many shear walls and masonry walls in each storey. In this analysis the rigidity of the secondary members (partition walls, masonry walls) were included. The fundamental period was 0.15 sec. for the longitudinal direction, and 0.18 sec. for the span. On the maximum earthquake response (recorded at Hachinohe Harbor) stresses of members (beams, columns, shear walls) were less than the cracking or shearing strength except the boundary beams (beams connected with the shear walls) in the span direction. Results of the analysis agreed with the observed damages of this building. The reasons why this building was only slightly damaged by the earthquake are, the shear walls had enough strength which was far than the designed shearing forces ; the second, shear walls are symmetrically aranged and disadvantage for torsion or local concentrated stressed could be avoided ; and the third, possible reduction of input earthquake force due to existence of the basement floor.