  In a general metric theory of gravitation in four dimensions, six
polarizations of a gravitational wave are allowed: two scalar and two vector
modes, in addition to two tensor modes in general relativity. Such additional
polarization modes appear due to additional degrees of freedom in modified
theories of gravitation or theories with extra dimensions. Thus, observations
of gravitational waves can be utilized to constrain the extended models of
gravitation. In this paper, we investigate detectability of additional
polarization modes of gravitational waves, particularly focusing on a
stochastic gravitational-wave background, with laser-interferometric detectors
on the Earth. We found that multiple detectors can separate the mixture of
polarization modes in detector outputs, and that they have almost the same
sensitivity to each polarization mode of stochastic gravitational-wave
background.
