An optical system design method determines a specific optical group having a relatively high decentering aberration error sensitivity to parallel decentering and tilt decentering, and minimizes the absolute value of the decentering aberration coefficient of that group. In a first stage, an optical system group consisting of at least one lens surface is identified which presents a problem in production because of a relatively high centering error sensitivity. A second stage calculates decentering aberration using a decentering aberration coefficient of the group identified in the first stage, and compares a decentering aberration that is obtained by the calculation with actual decentering aberration. A third stage identifies a third order aberration coefficient that makes a major contribution to increased centering error sensitivity when it is determined as the result of the comparison in the second stage that there is good agreement between the decentering aberration obtained by the calculation and the actual decentering aberration. A fourth stage minimizes the third order aberration coefficient identified in the third stage. A fifth stage restores an aberration balance that varied as a result of the design in the fourth stage, so that overall performance is maintained at a level equivalent to the condition before the design in the fourth stage.