Macroscopical and histological studies were carried out to clarify nerve innervation of the sacroiliac joint which is believed to have a role in the development of low back pain. Eighteen Japanese adult cadavers were used for gross anatomical examination and six for histological examination of the nerve supply to the joint. The results were as follows: 1) It is suggested by the gross examination that the upper ventral portion of the joint is mainly innervated by the ventral ramus of the 5th lumber nerve. 2) The lower ventral portion of the joint was mainly supplied by the ramus of the 2nd sacral nerve or branches from the sacral plexus. 3) Lateral branches of the dorsal ramus of the 5th lumber nerve were considered to be the main nerves which innervated the upper dorsal portion of the joint. 4) The lower dorsal portion was innervated by nerves arising from a plexus composed of lateral branches of the dorsal rami of the sacral nerves. 5) The nerves which innervate the joint range in diameter from 0.292 mm to 0.997 mm, and no difference was found among those in the four subdivisions of the joint. 6) Histological examination revealed that nerve fibers and the terminals were present in the joint capsule and adjoining ligaments. The nerve fibers varied from 0.2 micron to 2.5 microns in diameter and ended with five morphologically different terminals.