  We present optical linear spectropolarimetry of the microquasar LS I
+61$^{\circ}$ 303. The continuum emission is mildly polarized (up to 1.3 %) and
shows almost no temporal change. We find a distinct change of polarization
across the H$\alpha$ emission line, indicating the existence of polarization
component intrinsic to the microquasar. We estimate the interstellar
polarization (ISP) component from polarization of the H$\alpha$ line and derive
the intrinsic polarization component. The wavelength dependence of the
intrinsic component is well explained by Thomson scattering in equatorial disk
of the Be-type mass donor. The position angle (PA) of the intrinsic
polarization $\sim 25^{\circ}$ represents the rotational axis of the Be disk.
This PA is nearly perpendicular to the PA of the radio jet found during
quiescent phases. Assuming an orthogonal disk-jet geometry around the compact
star, the rotational axis of the accretion disk is almost perpendicular to that
of the Be disk. Moreover, according to the orbital parameters of the
microquasar, the compact star is likely to get across the Be disk around their
periastron passage. We discuss the peculiar circumstellar structure of this
microquasar inferred from our observation and possible connection with its
high-energy activities.

Astrophysics