We present a full spectropolarimetric study (in the four Stokes parameters I, Q, U and V) on Mira stars, and focus on the high level polarization detected in emission lines. From observations made with NARVAL instrument at TBL (Telescope Bernard Lyot) in Pic du Midi observatory (France), we confirm the early and single detection (McLean & Coyne, 1978) on Omicron Ceti - the prototype of Mira stars - when observed at its maximum luminosity : Balmer Hydrogen emission lines are strongly polarized, much more than the local contiunuum. Hence, for a sample of Mira stars, we are undergoing a complete characterization of the polarization in emission lines, with respect to the luminosity cycle. We find that this polarization in Balmer emission lines is likely to be linked with the emission line formation mechanism occurring within the de-excitation post shock wake of the strong shock wave propagating throughout the stellar atmosphere at each pulsation cycle. Moreover, according to shock wave theory, a magneto-electric field should appear just behind the shock front (where emission lines are formed), inducing polarization. To date, this field has never been conclusively detected on Mira stars (neither on any other type of radially pulsating star). Our spectropolarimetric observations constitute the first overall attempt to fully characterize and understand the associated physical mechanism.