  We study theoretically the effect of spin relaxation on the spin transport in
a ferromagnet/superconductor(FM/SC) tunnel junction. When spin-polarized
electrons are injected into the SC from the FM, nonequilibrium spin
accumulation as well as spin current are created in the range of the spin
diffusion length in the SC. We find that the spin diffusion length in the
superconducting state is the same as that in the normal state. We examine a
FM/SC/SC double tunnel junction, and show that the spin current is detected by
the Joule heat generated at the Josephson junction. This provides a method to
obtain the spin diffusion length by probing the spin current in SC's.
