After coming back to Japan, Saicho was highly interested in Mikkyo and came to study under Kukai, who was younger than him and lower than him in ranking as a Buddhist priest but studied authentic Mikkyo and came back to Japan.
帰国後も最澄の密教への関心は高く、自身より年少で僧としての地位も低いながら、正統的な密教を学んで帰国した空海に師事することになる。