A density measuring device includes a moving member that can be moved in a liquid, a light emitting member and a light receiving member that receives light emitted by the light emitting member. A gap part in which the moving member is disposed in a path of light emitted from the light emitting member can be moved. A density measuring unit measures the density of the liquid based on the result of output of the light receiving member for a case where the moving member is located in the path of light emitted from the light emitting member and a case where the moving member is not located in the path of light emitted from the light emitting member.