The structure of the mixed state of a current-carrying thin film of ideal type II superconductors in longitudinal magnetic field He is analysed. We show that the lossless current flow through the films in the mixed state is possible only if He is larger than Hc1)(d). The curves of longitudinal critical current obtained have complex structure and show peak effect in general. The main feature of the theoretical Ic-He curve of the films with thickness d larger than 5λ is identical with that observed by Heaton and Rose-Innes in Nb55Ta45 alloys.