Volume 1 of Thermae Romae (テルマエ・ロマエ) a manga series written and illustrated by Mari Yamazaki. It was serialized since 2008 in Comic Beam magazine from Enterbrain.
This manga series takes us to classic Rome, where an architect named Lucius has lost his inspiration. To find it, he goes to the spa, however, one day he will find a hidden tunnel that will bring him to a Japanese bathhouse.
Does not contain furigana.