A displacement detection apparatus (5) for a linear motion mechanism (3c) includes a nut member (36), a rod (2) including elongate grooves (2r, 2s) elongating in an axial direction of the rod (2) and a through hole (2h), a permanent magnet (5b) disposed parallel to the rod (2), a magnet block (50) made of synthetic resin and including a retaining portion (52) holding the permanent magnet (5b), a pair of leg portions (53) extending from ends of the retaining portion (52) that are spaced apart, and a nut (54) made of metal insert-molded at an intermediate position between the pair of leg portions (53), and a displacement sensor (5a) disposed opposing to the permanent magnet (5b). The pair of leg portions (53) and the nut (54) of the magnet block (50) are disposed in the elongate groove (2r) of the rod (2) and the magnet block (50) is fixed to the rod (2) by a bolt (51) screwed to the nut (54), the bolt (51) penetrating through the through hole (2h) extending through the rod (2).