A waste liquid treatment device (20) treats water to be treated that contains at least hydrogen peroxide, by decomposing the hydrogen peroxide. The waste liquid treatment device (20) is equipped with a housing (21), an introduction port (22) that is provided to the housing (21) and that introduces the water to be treated into the housing (21), a discharge port (23) that is provided to the housing (21) and that discharges treated water to be obtained by treating the water to be treated, and one or more channel-defining members (24) that are disposed within the housing (21) and that have a surface that a catalyst is disposed on, wherein the one or more channel-defining members (24) define, between the introduction port (22) and the discharge port (23), a channel (P) for the water to be treated, the channel (P) having a turning in at least one position.