A solenoid valve comprising: a valve sleeve of cylindrical shape having an inner passage continuing in an axial direction, a spool arranged in a movable manner along said axial direction in said inner passage of said valve sleeve, a center post of cylindrical shape having a through hole connected with said inner passage of said valve sleeve and constituted from a magnetic material, a plunger constituted from a magnetic material and arranged in a movable manner along said axial direction against said valve sleeve and said center post so as to be pulled towards said center post magnetized by the magnetic force generated by a coil, by placing said center post between said spool and said plunger in said axial direction, a shaft comprising an axis part and a collar part, said axis part having one end being contacted with said spool and other end attached to a plunger front end face of said plunger and moving in said axial direction inside said through hole of said center post, and said collar part protruding out in a radial direction from said axis part so as to be formed as one body with said axis part and sandwiched between said plunger front end face and a step face opposing said plunger front end face in said center post when said plunger closely approaches said center post, and a case part having said coil generating a magnetic force transferring to said center post and housing said center post, said plunger and said shaft inside, wherein a cross section outer peripheral shape of said axis part of said shaft comprises a plurality of arc shape portions arranged symmetrically against the center of said axis part, and a plurality of straight line notch portions connecting said plurality of arc shape portions, and the space formed between said straight line notch portion and an inner wall of said through hole becomes the flow passageway to flow the control fluid present around said plunger and inside said through hole, when said shaft moves in the axis direction, characterized in that said shaft is constituted from a non-magnetic material, and in that said axis part of said shaft comprises a plurality of larger diameter portions having said plurality of arc shape portions sliding against the inner wall of said through hole, and a smaller diameter portion arranged between said larger diameter portions at both sides in said axial direction and having maximum diameter smaller than said larger diameter portion in a cross section shape. a valve sleeve of cylindrical shape having an inner passage continuing in an axial direction, a spool arranged in a movable manner along said axial direction in said inner passage of said valve sleeve, a center post of cylindrical shape having a through hole connected with said inner passage of said valve sleeve and constituted from a magnetic material, a plunger constituted from a magnetic material and arranged in a movable manner along said axial direction against said valve sleeve and said center post so as to be pulled towards said center post magnetized by the magnetic force generated by a coil, by placing said center post between said spool and said plunger in said axial direction, a shaft comprising an axis part and a collar part, said axis part having one end being contacted with said spool and other end attached to a plunger front end face of said plunger and moving in said axial direction inside said through hole of said center post, and said collar part protruding out in a radial direction from said axis part so as to be formed as one body with said axis part and sandwiched between said plunger front end face and a step face opposing said plunger front end face in said center post when said plunger closely approaches said center post, and a case part having said coil generating a magnetic force transferring to said center post and housing said center post, said plunger and said shaft inside, wherein a cross section outer peripheral shape of said axis part of said shaft comprises a plurality of arc shape portions arranged symmetrically against the center of said axis part, and a plurality of straight line notch portions connecting said plurality of arc shape portions, and the space formed between said straight line notch portion and an inner wall of said through hole becomes the flow passageway to flow the control fluid present around said plunger and inside said through hole, when said shaft moves in the axis direction, characterized in that said shaft is constituted from a non-magnetic material, and in that said axis part of said shaft comprises a plurality of larger diameter portions having said plurality of arc shape portions sliding against the inner wall of said through hole, and a smaller diameter portion arranged between said larger diameter portions at both sides in said axial direction and having maximum diameter smaller than said larger diameter portion in a cross section shape.