The man is reported to be in a serious condition now. He was hospitalized on Friday among other 27 people injured in the fire that broke out in the clinic and was extinguished after burning across about 20 square meters (215 square feet).
The 61-year old is believed to have brought an inflammable liquid in a paper bag and ignited it early Friday, the sources said.
Police qualify the case as murder and arson and are trying now to identify 24 casualties.
The Japanese NHK broadcaster said Saturday, citing police, that about half an hour prior to the fire in the clinic, the suspect's house burnt too, presumably, due to arson. Investigative bodies are figuring out the connection between the two incidents.