In May 2017, a hen in a backyard chicken flock in Japan exhibited mild clinical signs, and the bird was examined for diagnosis. Unexpectedly, many microfilariae were observed in the lung by histologic examination, although no adult worms were detected within the body. In a blood test performed in July, microfilaremia was confirmed in a few clinically normal chickens of the same flock. Molecular analysis of the nematode partial 18S ribosomal RNA gene revealed that the gene detected in the lung of the necropsied hen was positioned in the group of the family Onchocercidae in the phylogenetic tree. These data show that avian filarioids that can infect chickens inhabit Japan.