A tool for cleaning inside of a water tank utilizing a siphon action is provided in which a scraping tool (S) is pivotably provided at an upstream end of a suction pipe (P), a deposit (b) such as moss caked on a bottom face or a side face within the water tank is scraped off by a scraping blade (62) formed at an extremity of the scraping tool (S), and is removed by sucking up together with dirt (a).