A cleaner for the inside of a water tank utilizing a siphoning action is provided, in which the interior of a water passage (24) between an upstream-side one-way valve (Vu) and a downstream-side one-way valve (Vd) is pressurized or depressurized by a pressing operation of a suction operation tool (A) provided in a bent connecting part between a main body portion (1) and a grip portion (2) of a suction pipe (P), and water-tank water of the main body portion (1) is sucked into the suction pipe (P). This enables waste material such as leftover food within water-tank water to be sucked up together with dirty water by a very simple operation.