Positive correlation between dust amount and star-formation activity in galaxies have been observationally reported in many previous works. Physical origin of this relation is not resolved quantitatively while many authors naively consider it as a result of the well established relation between gas density and surface SFR density (Kennicutt-Schdmidt law). We present a simple toy model for the relation between dust attenuation and galaxy global SFR (Av-to-SFR relation), based on the Kennicutt-Schdmidt law, gas-to-dust relation, mass-to-size relation, and galaxy mass-to-SFR relation (galaxy main sequence). Our model well reproduces the observed Av-to-SFR relations as well as their dispersion. Our analytic form will be a guide for works that need prior probability in Av-to-SFR space (e.g., in SED fitting analysis).