The sacroiliac joint (SIJ) can be a source of low back pain. Calcitonin gene-related peptide (CGRP) has been reported to play a significant role in nociceptive processing. However, the occurrence of CGRP-immunoreactive (CGRP-ir) sensory nerve fibers in the SIJ has not been fully defined. The present study investigated CGRP-ir nerve fibers supplying the SIJ. CGRP-ir nerve fibers in the vicinity of the SIJ cartilage and CGRP-ir neurons in the bilateral dorsal root ganglia (DRG) were examined immunohistochemically by administering anti-CGRP antiserum to rats. The SIJ was decalcified and cut into sections, and the CGRP-ir fibers around the SIJ cartilage were counted under microscopy. In another group, fluoro-gold (F-G), a neural tracer, was injected into the SIJ from the dorsal or ventral side with dorsal or ventral denervation. The number of F-G-labeled CGRP-ir neurons was counted in individual DRG. CGRP-ir fibers were observed more frequently in the tissues adjacent to the cranial part of the SIJ surface. In the case of dorsal denervation (ventral nerve supply), the CGRP-ir neurons composed 18.2% of the F-G-labeled neurons. In the case of ventral denervation (dorsal nerve supply), the CGRP-ir neurons composed 40.9% of the F-G-labeled neurons. There was a statistically significant difference in the number of CGRP-ir neurons between the ventral and dorsal nerve supplies to the SIJ. The cranial part of the dorsal side could be the part most associated with pain in the SIJ.