In a pump apparatus of the present invention, a portion of fluid sent to a
fluid-sending destination is circulated to a pumping driving chamber on
one side of a cylinder through a circulation passage. The pump apparatus
includes a switching valve which switches inflow and outflow of the
circulated fluid with respect to the pumping driving chamber. The
switching valve is connected to a piston rod through a link mechanism,
and mechanically carries out the switching operation using a pumping
driving force.