  For a simply connected solvable Lie group G with a cocompact discrete
subgroup {\Gamma}, we consider the space of differential forms on the
solvmanifold G/{\Gamma} with values in certain flat bundle so that this space
has a structure of a differential graded algebra(DGA). We construct Sullivan's
minimal model of this DGA. This result is an extension of Nomizu's theorem for
ordinary coefficients in the nilpotent case. By using this result, we refine
Hasegawa's result of formality of nilmanifolds and Benson-Gordon's result of
hard Lefschetz properties of nilmanifolds.
