  14 Ceti is a subgiant star of F spectral class that displays variations in
the S-index of its CaII H & K lines and an X-ray emission that is stronger than
the mean observed for its spectral class, which may be due to some magnetic
activity. We attempt to Zeeman-detect and study the magnetic field of 14 Ceti
and to infer its origin. We used the spectropolarimeter Narval at the Telescope
Bernard Lyot, Pic du Midi Observatory, and the least squares deconvolution
method to create high signal-to-noise ratio Stokes V profiles. We derived the
surface-averaged longitudinal magnetic field Bl. We also measured the S-index,
and the radial velocity for each observation. 14 Ceti is Zeeman-detected for
the 30 observed dates spanning from August 2007 to January 2012. The average
longitudinal magnetic field does not reverse its sign, reaches about -35 G, and
shows some month-long-timescale variations in our 2008 and 2011-2012
observations. The S-index follows the same long-term trend as Bl. 14 Ceti is
confirmed as a single star without H-K emission cores. The strength of the
observed surface magnetic field of 14 Ceti is one order of magnitude greater
than the observed one for late F main-sequence stars, and is comparable to the
values measured in the active late F pre-main-sequence star HR 1817. On the
other hand, taking into account the post-main-sequence evolution of an Ap star,
an oblique rotator model can explain the strength of the magnetic field of 14
Ceti. The variations with a timescale of months observed for both the Bl and
S-index could be due to the rotation. The most probable scenario to explain our
observations appears to be that 14 Ceti is the descendant of a cool Ap star .
