The innermost regions of quasars can be resolved by a gravitational lens ``telescope`` on scales down to a few AU. For this purpose, X-ray observations are most preferable because X-rays originating from the innermost regions can be selectively amplified by microlensing resulting from the ``caustic crossing.'' If detected, X-ray variations will constrain the size of the X-ray-emitting region down to a few AU. The maximum attainable resolution depends mainly on the monitoring intervals of lens events, which should be much shorter than the crossing time. On the basis of this idea, we performed numerical simulations of microlensing of an optically thick, standard-type disk as well as an optically thin, advection-dominated accretion flow (ADAF). Calculated spectral variations and light curves show distinct behaviors, depending on the photon energy. X-ray radiation that is produced in optically thin region exhibits intensity variation over a few tens of days. In contrast, optical-UV fluxes, which are likely to come from optically thick region, exhibit more gradual light changes, which is consistent with the microlensing events so far observed in Q2237+0305. Currently, Q2237+0305 is being monitored in the optical range at Apache Point Observatory. Simultaneous multiwavelength observations by X-ray satellites (e.g., ASCA, AXAF, XMM) as well as HST at the moment of a microlens event enable us to reveal an AU-scale structure of the central accretion disk around a black hole.