This invention relates to an apparatus for analyzing cells in a biological liquid sample, such as blood, in which apparatus a flow passage in a sample pretreatment system can be shortened with a simple construction. A sample metering chamber, a mixing chamber and a measurement liquid extracting chamber are formed in a rotary sliding member of a pretreatment portion. The rotary sliding member is intermittently rotated so as to shift a sample from one chamber to another to perform a necessary treatment. Fluorescence or scattering light with respect to the sample, mixed with a reagent in the pretreatment portion, is detected by a flow cell.