There are in fact two menhirs here. The first small, from which a signpost rises, and the second larger but fallen, to the west.
This is curious megalith country, very flat and in close proximity to the Loire River. Indeed I've just seen the vast bulk of the Queen Mary 2 in St.Nazaire ship works, where it is being built.
Why such a seemingly insignificant pair of menhirs warrant signposting from the main road I'm not sure, but if I must be the modern antiquarian then completion rules.