  Quite recently, a pronounced drop of $^{17}$O NMR Knight shift in the
superconducting (SC) state of an unstrained Sr$_2$RuO$_4$ was reported by
Pustogow and Luo ${\it et al.}$ They revealed such behavior from the
free-induction decay (FID) signal after a weak RF pulse. We examined this
behavior with our single-crystalline Sr$_2$RuO$_4$, and reproduced their
result: the peaks of the $^{17}$O-NMR spectra shift in the SC state as long as
the RF-pulse power is smaller than a threshold. Furthermore, we measured the
temperature variation of the Knight shift by a standard spin-echo method with
small-power RF pulses, and found that the spin susceptibility decreases in the
SC state. We conclude that our previous results of the invariance of the Knight
shift in the SC state were due to instantaneous destruction of
superconductivity by the RF pulses. The heat-up effect was characterized by the
temperature variation of the Knight shift under various measurement conditions.
