A biomolecule detector and a method thereof are provided to detect the biomolecules in a liquid sample using a field effect transistor (FET) array. The FET array is characterized in that the transistor used has no gate electrode layer, a reference electrode is provided in the space between the transistors in the array instead of the gate electrode layer. Using the FET array, the existence of the biomolecules in the sample can be detected electrically and effectively under the circumstance where an external voltage is applied to flow the biomolecules. Using the FET array to detect the biomolecules, the deviation for each transistor is reduced and the multiplex processing is also possible to measure the plurality of analyzing samples at the same time.