A micro object detection apparatus (11) includes an optical system (50). The first optical system (50) includes a first reflection region (101), a second reflection region (102), and a light reception element (6). The first reflection region (101) has an ellipsoidal shape, and reflects scattered light scattered when irradiation light hits a particle (R) to direct the scattered light to the light reception element (6), by utilizing two focal point positions of the ellipsoidal shape. The second reflection region (102) reflects scattered light coming from the particle (R) to direct the scattered light to the first reflection region (101), so that the scattered light is directed to the light reception element (6) by utilizing the ellipsoidal shape of the first reflection region (101). The light flux diameter of the scattered light reflected by the second reflection region (102) is larger than the particle (R), at the position of the particle (R) at which the scattered light is generated.