The object of this study is to make clear the relation between the shape of balcony and acoustical properties under the overhang, and to derive a design criterion for balcony configuration in auditoria. Impulse responses were calculated in models of auditoria by means of computer simulation based on geometrical acoustics when the depth and the height of balcony opening were varied. The directional characteristics of early reflections were examined in order to evaluate the lack of auditory envelopment, which was often perceived under the overhang. As a result, it was found that the ratio of the vertical component to total early reflection energy ER[inf V] was abruptly reduced under the balcony in comparison with that in the main orchestra and was closely related to the balcony index of d/h through the regression expression, where d/h was geometrically defined by the positions of sound source, receiving point, and balcony edge. Applying the difference limen of ER[inf V] to this relation, the maximum limit of d/h, within which the degree of auditory envelopment perceived under the overhang was equivalent to that in main orchestra, was obtained between 0.7 and 1.0.