The present invention relates to a solenoid valve configured to be mounted to a shock absorber of a vehicle to adjust damping force. solenoid valve of a shock absorber in which a damping force is adjusted through adjustment of back pressure against a disc valve. The solenoid valve includes a back pressure chamber for generating back pressure against the disc valve by a fluid introduced through a back pressure adjusting flow passage, and a spool moved in a spool guide by operation of a solenoid to control opening/closing or opening rate of the back pressure adjusting flow passage. The back pressure adjusting flow passage includes a variable orifice with an opening rate of the variable orifice being adjusted by the spool and the spool guide, the back pressure chamber, and a fixed orifice formed on the disc valve.