A sample analyzer is provided that is capable of eliciting sufficient performance of an avalanche photodiode, and performing high-precision analysis of the sample. The sample analyzer is provided with a WBC detection section that uses the avalanche photodiode as a photoreceptor element. The WBC detection section is provided with a flow cell, semiconductor light source, side collective lens, dichroic mirror, and avalanche photodiode. The side collective lens is a lens with a high NA (numeric aperture), and an aspheric lens with a small aberration. The sample analyzer prepares a scattergram using side fluorescent light signals and side scattered light signals to classify white blood cells into five categories.