A microparticle measuring apparatus which includes a flow channel through which a solution containing microparticles flows, an optical detecting unit configured to direct a laser beam to microparticles passing through the flow channel and detecting light for measurement emanating from the microparticles and converting the thus detected light into electrical signals, a solution feeding unit configured to feed the flow channel with either a sample solution containing microparticles of interest or a calibration solution containing reference microparticles that exhibit uniform optical characteristics, and an optical axis correcting unit configured to optimize the relative position of the flow channel with respect to the laser beam in response to the intensity of electrical signals from the reference microparticles.