Abstract: A method and an apparatus for detecting/quantifying bacteria in a sample rapidly and simply with sufficient sensitivity and accuracy by an enzyme activity method are provided. A test sample fluid is passed through a filter membrane 11 having a pore diameter of 0.6 to 5.0 ?m and/or a flow rate of distilled water passed of 50 to 500 mL/min·cm2 to collect the bacteria in the sample fluid on the filter membrane 11, and a lysing agent and an enzyme reaction substrate fluid are added to the bacteria collected on the filter membrane 11 to allow enzyme substrate reaction to proceed, and the enzyme activity is measured by a measuring device 33 to quantify the number of the bacteria in the sample fluid.