  Proposed space-based gravitational-wave detectors such as BBO and DECIGO can
detect ~10^6 neutron-star binaries and determine luminosity distance to the
binaries with a high precision. Combining the luminosity distance and
electromagnetically-derived redshift, one would be able to probe cosmological
expansion out to high redshift. In this paper, we show that the Hubble
parameter as a function of redshift can be directly measured with monopole and
dipole components of the luminosity distance on the sky. As a result, the
measurement accuracies of the Hubble parameter in each redshift bin up to z=1
are 3-14 %, 1.5-8 %, and 0.8-4% for the observation time 1 yr, 3 yr, and 10 yr,
respectively.
