A solenoid valve includes a housing having a cylindrical shape and having a valve path, a spool adjusting an opening degree of the valve path, a coil producing a magnetic force when a current passes therethrough, a first and a second stator core excited by the magnetic force of the coil, a shaft configured to be movable along axial direction together with the spool, a plunger fixed to the shaft and moving in the axial direction by adsorption force acting between itself and the first stator core by the magnetic force of the coil, and a coil spring biasing the spool against the adsorption force acting on the plunger. A spring constant of the coil spring increases as the adsorption force increases by the movement of the plunger.