Horizontally uneven wooden floor joists are each provided with a horizontal opening extending transversely thereof. A connecting rod member is inserted into such opening so that the ends of the rod protrude to both sides of the joist, the rod itself extending generally horizontally. The connecting rods are then subjected to tension forces generated by adjustable tension means of the type of a rod-and-buckle mechanism anchored to other connecting rod members provided in adjacent joists. The invention thus provides an extremely simply straightening method and apparatus avoiding the common cutting of high or low floor joists at their crest or dip and replacing the cut-out portion by a new joist.