Abstract: A fluorescence detection optical system comprises a light source which emits excitation light, a collimating lens which condenses the excitation light emitted from the light source into substantially parallel light, an objective lens which focuses the excitation light on a microchamber of a microfluidic device, an optical detector which measures an intensity of a fluorescence signal generated in the microchamber by the excitation light, a beam splitter which transmits or reflects the excitation light emitted from the light source toward the objective lens, and reflects or transmits the fluorescence signal generated in the microchamber toward the optical detector, and a beam shaping lens which is disposed between the beam splitter and the objective lens and expands an optical spot of the excitation light in one direction in accordance with a shape of the microchamber.