A field measurement was made at three stations near the head of Onagawa Bay to investigate the two-dimensional pattern of bay oscillations. Characteristics of the eigen oscillations are numerically calculated by finding the eigenvalues and eigenvectors of a matrix derived from a two-dimensional model. The observed oscillations can be explained, to a reasonable degree, by the calculated modes at the periods of 36, 18, 14, 12 and 10 min, where the modes at 18 and 12 min are of longitudinal oscillations and the ones at 14 and 10 min are of lateral oscillations.