The large ship recently built in Japan are generally of the type with the entire superstructure aft, and for these vessels special attention should be paid to vibration, especially to the fore-and-aft vibration of superstructures. The authors have carried out some experimental and theoretical investigation on these phenomena, and the following results have been obtained. (1) In full scale measurements, there were several peak groups recognized in the vicinities of 400600c/m and 8001, 000 c/m. of these, the lowest have possibility of resonating with the so-called blade frequencies. (2) Measured mode curves had shown interesting characteristics. (3) Some qualitative investigations have been made on these experimental data, and it becomes clear that the coupled motion of the main hull, the elastic support at the base deck of superstructure and the dogger have a great deal of influence upon the vibratory characteristics of superstructure. (4) A method of predicting the lowest natural frequency of superstructure by the use of electronic computer has been obtained.