  X-ray timing properties of the magnetar SGR 1900+14 were studied, using the
data taken with Suzaku in 2009 and NuSTAR in 2016, for a time lapse of 114 ks
and 242 ks, respectively. On both occasions, the object exhibited the
characteristic two-component spectrum. The soft component, dominant in energies
below $\sim 5$ keV, showed a regular pulsation, with a period of $P=5.21006$ s
as determined with the Suzaku XIS, and $P=5.22669$ with NuSTAR. However, in
$\gtrsim 6$ keV where the hard component dominates, the pulsation became
detectable with the Suzaku HXD and NuSTAR, only after the data were corrected
for periodic pulse-phase modulation, with a period of $T=40-44$ ks and an
amplitude of $\approx 1$ s. Further correcting the two data sets for complex
energy dependences in the phase-modulation parameters, the hard X-ray pulsation
became fully detectable, in 12-50 keV with the HXD, and 6-60 keV with NuSTAR,
using a common value of $T=40.5 \pm 0.8$ ks. Thus, SGR 1900+14 becomes a third
example, after 4U 0142+61 and 1E 1547$-$5408, to show the hard X-ray
pulse-phase modulation, and a second case of energy dependences in the
modulation parameters. The neutron star in this system is inferred to perform
free precession, as it is axially deformed by $\approx P/T =1.3 \times 10^{-4}$
presumably due to $\sim 10^{16}$ G toroidal magnetic fields. As a counter
example, the Suzaku data of the binary pulsar 4U 1626-67 were analyzed, but no
similar effect was found. These results altogether argue against the accretion
scenario for magnetars.
