The variable UV/optical emission results from the accretion disk reprocessing of the highly fluctuating X-ray emission. This can be tested by measuring inter-band time lags of quasars with different X-ray power. We report the inter-band time lag in an X-ray weak quasar, SDSS J153913.47+395423.4. We found a significant cross-correlation with a time delay of 32 days (observed-frame) detected in the Zwicky Transient Facility $g$ and $r$ light curves. During the talk, we will examine the required X-ray power in regard to the observed one and discuss its implications on the origin of quasar multi-band variability.