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_ c, at each frequency ν, and found a relation between W_ c and ν as W_ c(ν) ∝ν^-0.71±0.05. Then, by combining W_ c(ν) and the frequency dependence of the core position r_ c(ν), which was

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... tained by our previous study, we have constructed a collimation profile of the innermost jet W_ c(r) down to 10 Schwarzschild radii (R_ s) from the central black hole. We found that W_ 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_ 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.