The steering quality indices ( K : The index of turning ability, T : The index of stability on course and quickness in responding to steering) are normally obtained by fitting time histories of steerage and heading angle into Nomoto's 1^st order equation for maneuver. The drift angle, on the other hand, is sure to arise when a ship turns. If any analysis method for the steering indices and the drift angle is found out, it will make it easier to understand how large a ship laterally deviates especially in the stern. The focus on this paper is how to analyze the steering indices and the drift angle under the restriction of constant velocity and small heading angle. SR108 container ship is supposed as the analysis object ship and the steerage is considered to be closer to a practical maneuver rather than zig-zag one. The time histories of heading angle and trajectory are computed by the steerage supposed. The technical interest is whether the indices and drift angle could be calculated inversely from the time histories computed. The indices, in general, are obtained under the condition the steerage is known, but in this study, they are computed also in the condition it is unknown. The verification summaries that the indices are calculated inversely well if the steerage is known and even if it is unknown, trends of them can be grasped.