  We study magnetic field strengths along the jet in NGC~315. First, we
estimated the angular velocity of rotation in the jet magnetosphere by
comparing the measured velocity profile of NGC~315 with the
magneto-hydrodynamic jet model of proposed by Tomimatsu and Takahashi. Similar
to the case of M87, we find that the model can reproduce the logarithmic
feature of the velocity profile and suggest a slowly rotating black hole
magnetosphere for NGC~315. By substituting the estimated $\Omega_{F}$ into the
jet power predicted by the Blandford-Znajek mechanism, we estimate the magnetic
field strength near the event horizon of the central black hole as $5\times
10^{3}~{\rm G}\lesssim B_{H}\lesssim 2\times 10^{4}~{\rm G}$. We then estimate
magnetic-field strengths along the jet by comparing the spectral index
distribution obtained from VLBI observations with a synchrotron-emitting jet
model. Then we constrain the magnetic field strength at a de-projected distance
$z$ from the black hole to be in the range $0.06~{\rm G}\lesssim B(z)\lesssim
0.9~{\rm G}$ for $5.2 \times 10^{3}~r_{g}\lesssim z \lesssim 4.9 \times
10^{4}~r_{g}$, where $r_{g}$ represents the gravitational radius. By combining
the obtained field strengths at the event horizon and the downstream section of
the jet, we find that the accretion flow at the jet base is consistent with a
magnetically arrested disk (MAD). We discuss a comparison of the jet power and
the magnetic flux anchored to the event horizon in NGC~315 and M87.
