  It is well known that the perturbation equations of massless fields for the
Kerr-de Sitter geometry can be written in the form of separable equations. The
equations have five definite singularities so that the analysis has been
expected to be difficult. We show that these equations can be transformed to
Heun's equations, for which we are able to use known technique for the analysis
of the solutions. We reproduce results known previously for the Kerr geometry
and de Sitter geometry in the confluent limits of the Heun's functions. Our
analysis applies can be extended to Kerr-Newman-de Sitter geometry for massless
fields with spin 0 and 1/2.
