A numerical method is proposed for calculating the steady two-dimensional incompressible turbulent boundary layer with weak separated zones taken into account. The interaction between the inviscid flow and the boundary layer flow is expressed through the Bernoulli equation. The direct method, which does not use the interaction equation, reveals a singularity near the separation point. The interaction equation results in a smoothing of this singularity and enables one to calculate the flow upstream of the separation point. The method is extended to take into account heat transfer, so that the temperature distributions in the viscous region can be calculated even when kinematic boundary layer is fully developed.