The measurement of the 3-dimensional behavior of early reflections in a sound field has been an important issue in auditorium acoustics since the reflection profile has been found to be strongly correlated with the subjective responsiveness of a listener. In order to detect the incidence angle and relative amplitude of reflections, a 4-point microphone system has conventionally been used. A new measurement system is proposed in this paper, which has 5 microphones. Microphones are located on each four apex of a tetrahedron and at the center of gravity. Early reflections, including simultaneously incident reflections,which previous 4-point microphone system could not discriminate as individual wavefronts, were successfully found with the new system. In order to calculate accurate image source positions, it is necessary to determine the exact peak positions from measured impulse responses composed of highly deformed and overlapped impulse trains. For this purpose, a peak-detecting algorithm, which finds dominant peaks in the impulse response by an iteration method, is introduced. In this paper, the theoretical background and features of the 5-microphone system are described. Also, some results of experiments using this system are described.