The present invention has been made to solve the above problems, and accordingly an object thereof is to provide a looseness preventive nut which does not loosen even if vibrations are applied, and can be used as a single nut. In at least a first embodiment, the invention provides a looseness preventive nut 1 which is a nut threadedly engaging with a bolt 10, the nut being configured so that a lateral groove 3 extending in the radial direction in the range such that at least a part of a threaded portion 2 is cut, preferably in the range beyond the centerline C of the nut, is formed at a position slightly lower than a substantially middle point of the nut in the height direction.