A mesoscopic aluminum wire of 0.28 mu m width was prepared to investigate the resistance anomaly found in the superconducting transitions. We found that the radio-frequency (rf) radiation is an important factor in the anomaly, which strongly supports the recent work of Strunk et al. [Strunk, C., Bruyndoncx, V., Van Haesendonck, C., Moshchalkov, V. V., Bruynseraede, Y., Burk, B., Chien, C. -J. and Chandrasekhar, V., Phys. Rev. B, 1996, 53, 11332]. We performed a systematic study of the magnetic field dependence of the anomaly, which revealed the "mesa" type magnetoresistance near H = 0 for 1.13 K < T < 1.16 K. The boundary of the anomalous region in H-T space coincides with the upper critical field curve of the 2 mu m wide leads connected to the wire, implying a crucial role of the wide leads for the anomaly, having different T-c and dimensionality from those of the narrow wire. As soon as the leads become superconducting, the undissipated rf field transmits to the wire and induces the phase-slip centers to manifest the anomalous behavior. (C) 1998 Elsevier Science Ltd. All rights reserved.