This thesis starts with the Period of Saio Hamagutu Cabinet, for it is the best and accurate integration of Japanese Prewar Party Politics from prosperity to decline. Prototypical incident is represented by 1930 London Navy Treaty in this period. The development of Prewar Japanese Party Politics took a roundabout. It began with the Safeguarding Constitution Cabinet in 1924, but the Safeguarding Constitution Cabinet was defeated eight years later. The peak of this period from prosperity to decline was shown though the different attitude of Japanese various political cliques on the 1930 London Navy Treaty in this thesis.