Disclosed is a sample analyzer which comprises: a sample preparing section configured to prepare a measurement sample by mixing a sample and a nucleic acid staining reagent; an optical detector configured to irradiate light on cells contained in the measurement sample, receive fluorescent light given off by the irradiated cells, and output fluorescent light signals; a signal processing section which obtains fluorescent light intensity and fluorescence pulse area of the cells from the fluorescent light signals output by the optical detector; and an information processing section configured to detect white blood cells contained in the measurement sample based on the fluorescence pulse area, and detect bacteria contained in the measurement sample based on the fluorescent light intensity.