Kansan (year of birth and death unknown) was a monk during the Heian Period. There is a theory that he was an imaginary figure. His name was also written as 寛算 (Kansan and his usual name is written as 桓算).
He made an appearance in ancient works of the Heian Period such as "Okagami" (the Great Mirror) and the "Heike Monogatari" (The Tale of the Heike (Taira Clan)). Since he was also called 'Kansan gubu,' it is believed that he was the priest who served as naigubu (a special priest who held a position in the Imperial court). According to the "Okagami", he became a wraith who haunted Morosuke and his descendants after holding a grudge against FUJIWARA no Motokata and Morosuke. It is said that the eye disease of Emperor Sanjo was his fault, and the "Hobutsushu" (A Collection of Treasures) stated that lightning that fell on the Seiryoden (the residential palace of the emperor) in 930 was caused not by SUGAWARA no Michizane but by him.
However, there was no explanation given about how and why he became a wraith. According to "Shoyuki" (Diary of FUJIWARA no Sanesuke), the reason for the eye disease of Emperor Sanjo was the wraith of Gasho who could not become the head of the Tendai Sect after serving under FUJIWARA no Motokata as the naigubu (a special monk who held a position in the imperial court). Ryogen, who fought with Gasho for the position of head of the Tendai Sect and gained it, was the person who received the reverence of Morosuke and his descendants. There was a theory that stated that Kansan and Gasho were the same person, or that Kansan was an imaginary figure modeled after Gasho. However, the lightning that fell on Seiryoden and the death of FUJIWARA no Morosuke occurred while Gasho was alive, and if Gasho was the model for Kansan there must have been a great dramatization in future generations.