Abstract: A floor aligning jack for aligning higher and lower adjoining two floors of complementary halves of a structure such as a double wide mobile home. The aligning jack comprises a fastening bar for being fixedly secured to a band joist extending longitudinally along the length of the structure and to which the higher of the two floors of the complementary halves of the structure is secured. A shank is carried by the fastening bar for extending below the bottom of the band joists of both complementary structure halves and jacking means adjustably cooperate with the shank for being adjusted to engage the bottom of the band joist of the lower of the two floors. This raises and supports the lower of the two floors to the level of the higher floor.