A heel unit (10) of a safety ski binding comprises a unidirectional blocking arrangement (60') with at least one blocking member (62') arranged between a sole holder (16) and a spring biased return cam (26). The unidirectional blocking arrangement blocks in the opening direction and is releasable against a slight spring force (66) by means of a manual release member (64). One of the return cam (26) and the sole holder (16) is provided with a bearing face (68), generally circular or semicircular and concentric to the axis (A) of the return cam and of the sole holder for the blocking member (62') which is permanently resiliently urged into an actual blocking position.