This paper deals with the mechanism and relating matters on self-loosening rotation of bolted joints in the cases when very small slippages occur repeatedly at bearing surfaces under transverse loads. It can be supposed that accumulation of this kind of loosening finally produces serious states for bolted joints. Based on an assumption that a restoring force caused by an elastic torsion of a bolt shank which arises from a relative displacement between bolt and nut threads drives loosening rotation, a theoretical explanation is presented about how loosening rotation occurs and grows larger. Experiments in quasi-static states show results which agree with the theory of this paper. Additionally, consumption of transverse work and anti-loosening performance being taken up as relating subjects, more information about self-loosening is shown through examinations on some kinds of test samples.