Previously, we have studied the fingerprint-based indoor localization system using IEEE 802.15.4 wireless devices as low-power and compact ones suitable for continuous possessions of the users. For its practical applications, this FILS15.4 limits the detection resolution to one room in the target field. Unfortunately, human movements often cause the fluctuation of the receiving signal strength by affecting wireless propagation dynamics, which decreases the room detection accuracy of FILS15.4. To overcome the problem, in this paper, we investigate the detection accuracy improvement of FILS15.4, when the number of allocated receivers is increased in the field, and the number of fingerprints is increased for each room where the proper values are selected using the parameter optimization method. For the evaluation, we conducted extensive experiments using the FILS15.4 testbed system with three to five receivers at two floors of #2 Engineering Building in Okayama University. The results show that the use of five receivers achieves the accuracy up to 99.71% on average, whereas the use of three or four receivers does 82.64% and 97.37% respectively.