  The recent discovery of a radio-emitting neutron star with an ultralong spin
period of 76 s, PSR J0901-4046, raises a fundamental question on how such a
slowly rotating magnetized object can be active in the radio band. A canonical
magnetic field of $1.3\times10^{14}$ G estimated from the pulsar period and its
time derivative is wholly insufficient for PSR J0901-4046 to operate.
Consideration of a magnetic inclination angle of $10^\circ$ estimated from the
pulse width gives a higher magnetic field of $1.5\times10^{15}$ G, which is
still an order of magnitude lower than the necessary minimum of
$2.5\times10^{16}$ G following from the death line for radio pulsars with
magnetic fields exceeding the critical value $4.4\times10^{13}$ G. We show that
if the observed microstructure of single pulses reflects relativistic beaming,
the inferred surface magnetic field appears to be $3.2\times10^{16}$ G, and
without this assumption it is no less than $2.7\times10^{16}$ G, which explains
the existence of radio emission from PSR J0901-4046. This estimation makes PSR
J0901-4046 a radio pulsar with the strongest magnetic field known and is a sign
that PSR J0901-4046 slows down not by magnetic-dipole radiation, but rather by
an electric current of 56 MA, when rotational energy is expended in
accelerating charged particles over the polar cap.
