  This paper describes an analysis of the NuSTAR data of the fastest-rotating
magnetar 1E 1547$-$5408, acquired in 2016 April for a time lapse of 151 ks. The
source was detected with a 1-60 keV flux of $1.7 \times 10^{-11}$ ergs s$^{-1}$
cm$^{-2}$, and its pulsation at a period of $2.086710(5)$ sec. In 8-25 keV, the
pulses were phase-modulated with a period of $T=36.0 \pm 2.3$ ks, and an
amplitude of $\sim 0.2$ sec. This reconfirms the Suzaku discovery of the same
effect at $T=36.0 ^{+4.5}_{-2.5} $ ks, made in the 2009 outburst. These results
strengthen the view derived from the Suzaku data, that this magnetar performs
free precession as a result of its axial deformation by $\sim 0.6 \times
10^{-4}$, possibly caused by internal toroidal magnetic fields reaching $\sim
10^{16}$ G. Like in the Suzaku case, the modulation was not detected in
energies below $\sim 8$ keV. Above 10 keV, the pulse-phase behaviour, including
the 36 ks modulation parameters, exhibited complex energy dependences: at $\sim
22$ keV, the modulation amplitude increased to $\sim 0.5$ sec, and the
modulation phase changed by $\sim 65^\circ$ over 10--27 keV, followed by a
phase reversal. Although the pulse significance and pulsed fraction were
originally very low in $>10$ keV, they both increased noticeably, when the
arrival times of individual photons were corrected for these systematic
pulse-phase variations. Possible origins of these complex phenomena are
discussed, in terms of several physical processes that are specific to
ultra-strong magnetic fields.

High Energy Astrophysical Phenomena