This paper reports on a flexible flap valve actuated by electromagnetic force under a constant pressure source. The flexible flap valve consists of the three main components: a flexible flap with a steel disk embedded in PDMS, an electromagnetic actuator and two glass plates with inlet and outlet. The flap valve is fabricated by SU-8 mold process, the EDM process and oxygen plasma treatment. The dimension of an assembled flap valve is $12[mm]{ imes}20[mm]{ imes}28[mm]$. The stroke volume of the flap valve is measured for various pressures. And the time of the applied input voltage is varied to change the open time of the valve. When the input voltage of 30[V] is applied for 0.25[s], the minimum stroke volume of the flap valve is $40[{mu}L]$ at 70 [kPa].