An antigen detection method using sandwich immunoassay, comprising at least: providing a sensor chip, which comprises at least a reaction zone where an antigen-capturing antibody is immobilized in the middle of a fine flow channel, a liquid discharge/suction section on the upstream side of the fine flow channel and a liquid-mixing section on the downstream side of the fine flow channel, wherein the liquid-mixing section has a special form so configured as to generate turbulence in the liquid-mixing section, wherein in this form, the liquid-mixing section is formed by a gently-sloping surface and a vertical surface that is arranged in an erected manner in continuation from the sloping surface, such that the turbulence is generated when a liquid is fed until the liquid reaches the liquid-mixing section; a first reaction step of feeding a sample liquid containing a target antigen to the reaction zone of the sensor chip so as to allow the antigen-capturing antibody to capture the target, and the sample liquid is fed until the sample liquid reaches the liquid-mixing section arranged on the downstream side of the fine flow channel; a second reaction step of allowing a labeling liquid containing a labeling antibody to flow into the fine flow channel so as to label the target captured by the antigen-capturing antibody, after the first reaction step, wherein the labeling liquid is fed in such a manner that the labeling liquid does not reach the liquid-mixing section arranged on the downstream side of the fine flow channel; and a signal measurement step of measuring a signal obtained from the labeling antibody used for labeling the target in the second reaction step; wherein the sample liquid and the labeling liquid are fed via the same liquid discharge/suction section. providing a sensor chip, which comprises at least a reaction zone where an antigen-capturing antibody is immobilized in the middle of a fine flow channel, a liquid discharge/suction section on the upstream side of the fine flow channel and a liquid-mixing section on the downstream side of the fine flow channel, wherein the liquid-mixing section has a special form so configured as to generate turbulence in the liquid-mixing section, wherein in this form, the liquid-mixing section is formed by a gently-sloping surface and a vertical surface that is arranged in an erected manner in continuation from the sloping surface, such that the turbulence is generated when a liquid is fed until the liquid reaches the liquid-mixing section; a first reaction step of feeding a sample liquid containing a target antigen to the reaction zone of the sensor chip so as to allow the antigen-capturing antibody to capture the target, and the sample liquid is fed until the sample liquid reaches the liquid-mixing section arranged on the downstream side of the fine flow channel; a second reaction step of allowing a labeling liquid containing a labeling antibody to flow into the fine flow channel so as to label the target captured by the antigen-capturing antibody, after the first reaction step, wherein the labeling liquid is fed in such a manner that the labeling liquid does not reach the liquid-mixing section arranged on the downstream side of the fine flow channel; and a signal measurement step of measuring a signal obtained from the labeling antibody used for labeling the target in the second reaction step; wherein the sample liquid and the labeling liquid are fed via the same liquid discharge/suction section.