An ultra small fluorescence detector capable of detecting in real time reaction undergoing in a micro chamber having a predetermined volume and disposed on a microfluid chip is provided. The fluorescence detector for detecting in real time PCR amplification undergoing in the microfluid chip having a micro chamber with a predetermined volume includes a light source generating an excitation beam, afirst optical system capable of irradiating the excitation beam having a predetermined spot size to the micro chamber, a first detector, and a second optical system reflecting a fluorescent beam derived from the excitation beam having the predetermined spot size in the micro chamber to the first detector. Accordingly, the fluorescence detector is designed such that light emitted by a light sourceis focused between a first mirror and an objective lens. Therefore, the spot size of an excitation beam transmitted by the objective lens is largely formed so that the excitation beam can be irradiated on the whole micro chamber of the microfluid chip, thereby detecting a fluorescent beam on a broader area.