An examination apparatus for estimating a population of microorganisms contained and living in ballast water discharged from a ship from a number of microorganisms present in ballast water contained in a sample container and taken as a sample and for enabling determination whether or not the number of microorganisms contained in the ballast water meets ballast water discharge standards, the apparatus comprising: stirring and mixing means comprising a sample container formed of a material allowing light to pass through, for stirring and mixing the sample solution in the sample container; an excitation light source that irradiates the sample container with excitation light; light receiving means for detecting light and converting the light into an electric signal; control means for calculating the amount of the microorganisms contained in a sample in the sample container to enable determination whether or not ballast water discharge standards are met; and filtering means between the light receiving means and the control means, wherein the sample solution is prepared by adding a fluorescent staining reagent that stains the microorganisms to the sample, the light receiving means detects a fluorescent emission from the sample solution resulting from irradiation with the excitation light from the excitation light source, and the control means detects the number of emissions based on the electric signal from the light receiving means to calculate the amount of the microorganisms contained in the sample in the sample container; and the filtering means filters out noise of a low frequency component and a noise of a high frequency component contained in the electric signal from the light receiving means, wherein an operation section including a plurality of operating buttons is electrically connected to the control means, and wherein the operation section includes a setting button that enables switching between different sizes of microorganisms to be measured. stirring and mixing means comprising a sample container formed of a material allowing light to pass through, for stirring and mixing the sample solution in the sample container; an excitation light source that irradiates the sample container with excitation light; light receiving means for detecting light and converting the light into an electric signal; control means for calculating the amount of the microorganisms contained in a sample in the sample container to enable determination whether or not ballast water discharge standards are met; and filtering means between the light receiving means and the control means, wherein the sample solution is prepared by adding a fluorescent staining reagent that stains the microorganisms to the sample, the light receiving means detects a fluorescent emission from the sample solution resulting from irradiation with the excitation light from the excitation light source, and the control means detects the number of emissions based on the electric signal from the light receiving means to calculate the amount of the microorganisms contained in the sample in the sample container; and the filtering means filters out noise of a low frequency component and a noise of a high frequency component contained in the electric signal from the light receiving means, wherein an operation section including a plurality of operating buttons is electrically connected to the control means, and wherein the operation section includes a setting button that enables switching between different sizes of microorganisms to be measured.