To see how superconducting gap structures affect the longitudinal component of mixed-state thermal conductivity K-xx(B), the magnetic-field dependences of K-xx(B) in s- 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 K-xx(B) measurement as a method of probing the gap structure.