The magnetic properties of triangle-based spin ladder system are investigat
ed for (C1TET-TTF)(FeBr4)(1-x)(FeCl4)(x), in which the ratio of the strengt
hs of two constituent interactions varies depending on x. A spin-canting we
ak ferromagnetic state is stabilized for x=0 and 1 where the spin easy axis
is oriented along different directions between x=0 and 1. In the intermedi
ate range x=0.18, a spiral spin arrangement appears, consistent with theore
tical suggestion for the triangle-based spin ladder systems.