During the 'Yugao' chapter (The Tale of Genji), the chapter in which he becomes a central character, he holds the position of Tono Chujo, so this is how he was known by later readers.
彼が重要人物となる「夕顔(源氏物語)」巻での官職が頭中将であったため、後世の読者からこう呼ばれている。