  The explanation of the nature of superconducting gap in high temperature
superconductors (HTSC) is a fundamental task which solution can lead to the
understanding of superconducting mechanism. However, it has not been fully
solved yet. From the mid of the twentieth century when Bardeen, Cooper and
Schrieffer constructed their theory it has been believed that a superconducting
gap is a collective phenomenon of electron excitations. In this work it is
demonstrated that according to translation-invariant bipolaron theory of HTSC
the different types of experiments measure for the gap different values. Thus
tunneling experiments determine the bipolaron energy for a superconducting gap.
On the other hand, the angle - resolved photoemission spectroscopy method
measures the phonon frequency for which the electron-phonon interaction is
maximum. Such effects as kinks in spectral measurements of gap, its angular
dependence, existence of pseudogap and others have got natural explanations.
