The static instability of the front-free-aft-fixed rectangular elastic plate in the supersonic uniform airstream is studied analytically. The two-dimensional supersonic small perturbation potential theory is used for the aerodynamic force evaluation. The aeroelastic divergence speed and the divergence mode are obtained by the analysis. And the piston theory is used also for the aerodynamic force evaluation. The divergence speed based on the small perturbation potential theory is coincident with the one based on the 2nd order piston theory, when the flow Mach number is higher than 3.