A detector comprising: a channel for flowing a sample liquid; and magnetic field generating means, which is movable to directly below the channel, by which a magnetic particle bound to a specific substance in the sample liquid is captured by magnetic attraction between the magnetic particle and the magnetic field generating means in a measurement region of the channel, wherein the magnetic field generating means is disposed with two surfaces (A,B) having magnetic poles being perpendicular to the direction of the flow in the channel, and two surfaces (C,D) having no magnetic poles being parallel to the direction of the flow in the channel, wherein the magnetic field generating means is movable away from the channel so that no magnetic field is applied to the channel, the detector further comprising a laser light source that is configured to illuminate the magnetic particle remaining in the measurement region when no magnetic field is applied to the channel, wherein the channel is configured so that the magnetic particles spread in a planar fashion as they travel along a gradually widening path inside the detection channel until they are captured in the measurement region under the magnetic force of the magnetic field generation means; the channel having a maximum channel width b at the measurement region in a direction perpendicular to the flow direction of the channel; characterized in that the width a of the magnetic field generating means, which is the distance between the surfaces (C,D) having no magnetic poles, in the direction perpendicular to the flow direction of the channel, and the maximum channel width b of the channel at the measurement region in the direction perpendicular to the flow direction of the channel, satisfy the ratio a / b of 1.11 ≤ a / b ≤ 1.85. a channel for flowing a sample liquid; and magnetic field generating means, which is movable to directly below the channel, by which a magnetic particle bound to a specific substance in the sample liquid is captured by magnetic attraction between the magnetic particle and the magnetic field generating means in a measurement region of the channel, wherein the magnetic field generating means is disposed with two surfaces (A,B) having magnetic poles being perpendicular to the direction of the flow in the channel, and two surfaces (C,D) having no magnetic poles being parallel to the direction of the flow in the channel, wherein the magnetic field generating means is movable away from the channel so that no magnetic field is applied to the channel, the detector further comprising a laser light source that is configured to illuminate the magnetic particle remaining in the measurement region when no magnetic field is applied to the channel, wherein the channel is configured so that the magnetic particles spread in a planar fashion as they travel along a gradually widening path inside the detection channel until they are captured in the measurement region under the magnetic force of the magnetic field generation means; the channel having a maximum channel width b at the measurement region in a direction perpendicular to the flow direction of the channel; characterized in that the width a of the magnetic field generating means, which is the distance between the surfaces (C,D) having no magnetic poles, in the direction perpendicular to the flow direction of the channel, and the maximum channel width b of the channel at the measurement region in the direction perpendicular to the flow direction of the channel, satisfy the ratio a / b of 1.11 ≤ a / b ≤ 1.85.