Abstract: Based on the ideal magnetohydrodynamics, the present paper discusses the linear stability of a semicircular electric current sheet tied at the photoshere. The main conclusion is that the electric current sheet becomes unstable if the magnetic field component in the direction of the electric current is less than a certain critical value. Furthermore, the wave number of the disturbance in the current direction, which causes the instability first, is approximately the reciprocal of the diameter of the current sheet circle.