We examine the correlation between the star formation rate (SFR) and the strength of mean interstellar magnetic fields, adopting some simple assumptions. According to the recent work by Vallee, the strength of an interstellar magnetic field (B) have only weak dependence on SFR, and he observationally obtained B ~ SFR0.13. However, adopting two assumptions (the so-called Schmidt law and double equipartition of energy among three types of interstellar medium [ISM] [thermal gas: Eth, magnetic fields: Emag, and nonthermal component: Enon]), we find that Vallee's observational result includes an important meaning. According to our result, the correlation predicted by the double equipartition theory is represented by B ~ SFR1/2n where n is defined by SFR ~ rho n. We note that Vallee adopts the data corresponding to the case of single equipartition (Emag = Enon) and n = 1.3. Then, B should have a stronger dependence on SFR as far as our assumptions permit. This discrepancy suggests that the double equipartition model of ISM is not established in a galactic wide view (Emag ≠ Eth).