A microorganism detection apparatus including a light source unit to irradiate light, a mirror chamber including a light inlet port and a light outlet port, through which the light is introduced and discharged, respectively an inlet port and an outlet port, through which particles are introduced and discharged, respectively, so that light emitted from the particles (emitted particle light) is generated by the light, and an opening, through which the emitted particle light is discharged outside. The mirror chamber has an oval longitudinal section, and is provided at an inside thereof with a mirror. A condensing optical system is disposed in front of the opening outside the opening to condense the emitted particle light discharged through the opening. Condensing efficiency of the fluorescent light condensing optical system is maximized.