  The present paper reports on the RXTE observations of the binary X-ray pulsar
4U0115+63, covering an outburst in 1999 March-April with 44 pointings. The 3-30
keV PCA spectra and the 15-50 keV HEXTE spectra were analyzed jointly for the
cyclotron resonance features. When the 3-50 keV luminosity at an assumed
distance of 7 kpc was in the range (5-13)x10^{37} erg s^{-1}, harmonic double
cyclotron features were observed in absorption at ~11 and ~22 keV, as was
measured previously during typical outbursts. As the luminosity decreased below
\~5x10^{37} erg s^{-1}, the second resonance disappeared, and the fundamental
resonance energy gradually increased, up to $\sim$16 keV at 0.16x10^{37} erg
s^{-1}. These results reconfirm the report by Mihara et al. (2004) using Ginga,
who observed a single absorption at ~16 keV in a minor (~10^{37} erg s^{-1})
outburst of this object. The luminosity-dependent cyclotron resonance energy
may be understood as a result of a decrease in the accretion column height, in
response to a decrease in the mass accretion rate.
