We report on the first birds-eye view of the innermost accretion disk around the high-mass protostellar object G353.273+0.641, taken by Atacama Large Millimter/submillimeter Array long-baselines. The disk traced by dust continuum emission has a radius of 250 au, surrounded by the infalling rotating envelope traced by thermal CH$_3$OH lines. This disk radius is consistent with the centrifugal radius estimated from the specific angular momentum in the envelope. The lower-limit envelope mass is $sim$5-7 M$_{odot}$ and accretion rate onto the stellar surface is 3 $times$ 10$^{-3}$ M$_{odot}$ yr$^{-1}$ or higher. The expected stellar age is well younger than 10$^{4}$ yr, indicating that the host object is one of the youngest high-mass objects at present. The disk mass is 2-7 M$_{odot}$, depending on the dust opacity index. The estimated Toomres $Q$ parameter is typically 1-2 and can reach 0.4 at the minimum. These $Q$ values clearly satisfy the classical criteria for the gravitational instability, and are consistent with the recent numerical studies. Observed asymmetric and clumpy structures could trace a spiral arm and/or disk fragmentation. We found that 70$%$ of the angular momentum in the accretion flow could be removed via the gravitational torque in the disk. Our study has indicated that the dynamical nature of a self-gravitating disk could dominate the early phase of high-mass star formation. This is remarkably consistent with the early evolutionary scenario of a low-mass protostar.
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