To rationalize form construction work, thermal insulation material mounting construction work, and waterproofing construction work when a water tank is constructed.

This placing form 1 for constructing a concrete water tank is constituted by laminating a waterproof sheet 3 integrally on the whole surface of a panel body 2 made of a thermal insulation material. A fringe part of the waterproof sheet 3 is extended to the outside of the panel body 2 as a connection piece 3a for overlapping it on the waterproof sheet 3 of the other placing form 1 to be installed adjacent to it. A heat weldable sheet material is used as a raw material of the waterproof sheet 3. Polypropylene sheet is used as the waterproof sheet 3, and foaming polypropyrene is used as a raw material of the panel body 2.