This report discusses about the “multicollinearity” problem in the equation error method. This problem represents the limit of the aerodynamic characteristics information given by the flight-test data. This problem is occurred not only when the strong correlations between the explanatory variables are generated by the aircraft's maneuver, but also when the number of the explanatory variables is more than the maximum number of the variables which can be essentially independent in the equation of motion. Furthermore, the physical meanings about the partial regression coefficients and the multicollinearity are schematically explained with the regression plane.