  We investigated the detailed inner jet structure of M87 using the Very Long
Baseline Array data at 2, 5, 8.4, 15, 23.8, 43, and 86 GHz, especially focusing
on the multi-frequency properties of the radio core at the jet base. First, we
measured a size of the core region transverse to the jet axis, defined as
$W_{\rm c}$, at each frequency \nu, and found a relation between $W_{\rm c}$
and $\nu$ as $W_{\rm c}(\nu) \propto \nu^{-0.71\pm0.05}$. Then, by combining
$W_{\rm c}(\nu)$ and the frequency dependence of the core position $r_{\rm
c}(\nu)$, which was obtained by our previous study, we have constructed a
collimation profile of the innermost jet $W_{\rm c}(r)$ down to ~10
Schwarzschild radii ($R_{\rm s}$) from the central black hole. We found that
$W_{\rm c}(r)$ smoothly connects with the width profile of the outer
edge-brightened, parabolic jet, and then follows a similar radial dependence
down to several tens of $R_{\rm s}$. Closer to the black hole, the measured
radial profile suggests a possible change of the jet collimation shape from the
outer parabolic one, in which the jet shape tends to become more
radially-oriented. This could be related to a magnetic collimation process
or/and interaction with surrounding materials at the jet base. The present
results shed light on the importance of higher-sensitivity/resolution imaging
studies for M87 at 86, 43 and also 22 GHz, and should be examined more
rigorously.
