  In-plane magnetoresistance for organic massless Dirac electron system (OMDES)
$\alpha$-(BEDT-TTF)$_2$I$_3$ and $\theta$-(BEDT-TTF)$_2$I$_3$ in addition to
possible candidates of the OMDES $\alpha$-(BETS)$_2$I$_3$ and
$\alpha$-(BEDT-STF)$_2$I$_3$, was investigated under hydrostatic pressure. We
have found the universal behavior of the in-plane magnetoresistance under a low
magnetic field perpendicular to two-dimensional plane. As for
$\alpha$-(BEDT-TTF)$_2$I$_3$, the universality was examined with the parameters
of temperature, magnetic field and its direction. We suggest that the universal
magnetoresistance behavior is found even for the gapped state of
$\alpha$-(BEDT-TTF)$_2$I$_3$ under intermediate pressure, when the thermal
energy exceeds the gap.
