  We report x-ray diffraction, resistivity, thermopower, and magnetization of
Sr3ErMn4-xGaxO10.5-d, in which A-site ordered tetragonal phase appears above
x=1, and reveal that the system exhibits typical properties seen in the
antiferromagnetic insulator with Mn3+. We succeed in preparing both A-site
ordered and disordered phases for x=1 in different preparation conditions, and
observe a significant decrease of the resistivity in the disordered phase. We
discuss possible origins of the decrease focusing on the dimensionality and the
disordered effect.
