[Problem] To provide, inter alia, a desired-substance detection device that can be used in detecting a desired substance in which magnetic particles are used, and that can be manufactured inexpensively and compactly. [Solution] This desired-substance detection device is characterized in comprising a detection chip 1, a light radiation unit B, and a magnetic field application unit C, where: the detection chip 1 comprises a substantially plate-form optically transparent member 2 that has a detection surface 2a disposed on the upper-surface side of a bottom surface, an upward-inclined surface 2b that is inclined away from the detection surface 2a from the thickness-direction upper surface toward the bottom-surface side, and a body part 2c configured to be capable of receiving light and guiding the light through the interior, the optically transparent member 2 having an optical impingement structure in which the light radiated from the upper-surface side and transmitted through the upward-inclined surface 2b is a
