A spool position detection device for a flow rate control valve that includes a spool having a plurality of valve members and slidably accommodated in a sleeve having a plurality of ports (P1 - P4), said flow rate control valve performing flow rate control by linearly moving the spool by means of an actuator, said spool position detection device comprising: a straight target probe formed of a magnetically-responsive substance and configured to be mounted to one end of the spool, the sleeve having an opening formed in one end thereof to permit passage therethrough of said target probe; a sensor housing configured to be mounted to the one end of the sleeve for detecting a linear position of said target probe, said sensor housing having a cylinder section formed of a non-magnetic substance and defining an inner space to permit entry therein of said target probe, the inner space being configured to communicate with the opening formed in the one end of the sleeve; and first and second coils (C1, C2) disposed around the cylinder section within said sensor housing and axially spaced from each other by a predetermined distance, said first coil being disposed so as to respond to said target probe, said target probe being constructed in such a manner that magnetic response of said first coil gradually varies in one direction in response to a linear position of said target probe, said second coil being constructed so as not to respond to linear displacement of said target probe at least within a predetermined linear displacement range of said target probe, wherein a linear position of said target probe, and hence a position of the spool, is detected based on calculating a difference between outputs of the first and second coils (C1, C2), and wherein said first coil is located nearer to said target probe than said second coil, characterized in that said target probe has a length such that a distal end thereof can enter a sensitive region of said second coil, and that said target probe has such a shape that a predetermined portion of said target probe close to a proximal end thereof presents a greater gradual volume change rate, so that, when the distal end thereof has entered the sensitive region of said second coil, the predetermined portion of said target probe enters a sensitive region of said first coil and enhances a degree of sensitivity of said first coil so as to cancel out sensitivity of said second coil by means of the predetermined portion close to the proximal end. a straight target probe formed of a magnetically-responsive substance and configured to be mounted to one end of the spool, the sleeve having an opening formed in one end thereof to permit passage therethrough of said target probe; a sensor housing configured to be mounted to the one end of the sleeve for detecting a linear position of said target probe, said sensor housing having a cylinder section formed of a non-magnetic substance and defining an inner space to permit entry therein of said target probe, the inner space being configured to communicate with the opening formed in the one end of the sleeve; and first and second coils (C1, C2) disposed around the cylinder section within said sensor housing and axially spaced from each other by a predetermined distance, said first coil being disposed so as to respond to said target probe, said target probe being constructed in such a manner that magnetic response of said first coil gradually varies in one direction in response to a linear position of said target probe, said second coil being constructed so as not to respond to linear displacement of said target probe at least within a predetermined linear displacement range of said target probe, wherein a linear position of said target probe, and hence a position of the spool, is detected based on calculating a difference between outputs of the first and second coils (C1, C2), and wherein said first coil is located nearer to said target probe than said second coil, characterized in that said target probe has a length such that a distal end thereof can enter a sensitive region of said second coil, and that said target probe has such a shape that a predetermined portion of said target probe close to a proximal end thereof presents a greater gradual volume change rate, so that, when the distal end thereof has entered the sensitive region of said second coil, the predetermined portion of said target probe enters a sensitive region of said first coil and enhances a degree of sensitivity of said first coil so as to cancel out sensitivity of said second coil by means of the predetermined portion close to the proximal end.