In a density detection device, a casing includes an internal space. An inflow path is communicated with the internal space through an inlet facing the internal space. An outflow path is communicated with the internal space through an outlet facing the internal space. A base is provided in the internal space to block the liquid flowing from the inlet. The base has a top side positioned above the inlet. A movable member has a bottom side opposed to the top side of the base. The movable member is configured to move to cause the bottom side to be close to and away from the top side of the base. A density detection section is configured to detect density of a liquid layer formed between the bottom side and the top side while the bottom side is positioned close to the top side at predetermined distance.