A hydrothermal treatment device (3) is a hydrothermal treatment device (3) performing hydrothermal treatment by heating high-water-content biomass, the hydrothermal treatment device (3) including a treatment container (21) that stores sludge, a sludge supply unit (22) that supplies the sludge to inside of the treatment container (21) such that a space (S) is formed in a vertical upper part of the treatment container (21), a stirrer (23) that is provided within the treatment container (21) and stirs stored matter such that counter flows in an up/down direction occur, and a heat transfer tube (24) that is disposed in a horizontal direction within the treatment container (21) and heats the sludge with heat of vapor flowing within the heat transfer tube (24).