  To see how superconducting gap structures affect the longitudinal component
of mixed-state thermal conductivity kappa_{xx}(B), the magnetic-field
dependences of kappa_{xx}(B) in s-wave and d-wave superconductors are
investigated. Calculations are performed on the basis of the quasi-classical
theory of superconductivity by fully taking account of the spatial variation of
the normal Green's function, neglected in previous works, by the
Brandt-Pesch-Tewordt approximation. On the basis of our result, we discuss the
possibility of kappa_{xx}(B) measurement as a method of probing the gap
structure.
