Because the induced dipole moment due to polarizability in a moderate electric field is thought to be small, the Stark effect due to polarizability has generally been ignored for molecules that have a permanent electric dipole moment. However, by observation of the Stark modulation spectra (SMS), we have confirmed that the effects of polarizability and of the permanent dipole moment are sometimes comparable. Based on this observation, we have sought an estimate of the polarizability. It was also found that the effect of the polarizability on the SMS depends mainly on its isotropic part. Thus, for the B-X electric transition of the ICl molecule, the differences in isotropic polarizabilities were estimated to be Delta<(alpha)over bar> = 3.4-3.8 Angstrom(3) at the 3-1 transition, and Delta<(alpha)over bar> = 3.6-4.3 Angstrom(3) at the 1-0 transition, assuming that the differences of anisotropic polarizability Delta alpha(an) are within the range 2-9 Angstrom(3) for both transitions. Thus, obtained values of Delta<(alpha)over bar> were confirmed to be in a reasonable range, in spite of a wide range uncertainty in Delta alpha(an).