A support device for supporting beams of formwork for floor slabs, which comprises an upright, the end of which can be fixed to a supporting prop that rests on the ground; a supporting head for at least one supporting beam of floor slab formwork, which is arranged coupled slideably along the upright, and elements of reversibly locking the supporting head on a preset position defined along the upright, the supporting head in turn comprises: two plates, at least two recesses, for guiding insertion and for containment for a resting portion of the corresponding supporting beam, each one of which is provided on a respective plate, each recess having a substantially flat bottom with a breadth such as to define a variable resting point for the supporting beam along the bottom as a function of the inclination of the supporting beam.