  We report polarization measurements in two prompt emissions of gamma-ray
bursts, GRB 110301A and GRB 110721A, observed with the Gamma-ray burst
polarimeter (GAP) aboard IKAROS solar sail mission. We detected linear
polarization signals from each burst with polarization degree of $\Pi = 70 \pm
22$% with statistical significance of $3.7 \sigma$ for GRB 110301A, and $\Pi =
84^{+16}_{-28}$% with $3.3 \sigma$ confidence level for GRB 110721A. We did not
detect any significant change of polarization angle. These two events had
shorter durations and dimmer brightness compared with GRB 100826A, which showed
a significant change of polarization angle, as reported in Yonetoku et al.
(2011). Synchrotron emission model can be consistent with all the data of the
three GRBs, while photospheric quasi-thermal emission model is not favorable.
We suggest that magnetic field structures in the emission region are
globally-ordered fields advected from the central engine.
