Provided is a beam irradiation apparatus including: a beam scanner that is configured such that a charged particle beam is reciprocatively scanned in a scanning direction; a measurement device that is capable of measuring an angular component of charged particles incident into a region of a measurement target; and a data processor that calculates effective irradiation emittance of the charged particle beam using results measured by the measurement device. The measurement device measures a time dependent value for angular distribution of the charged particle beam. The data processor transforms time information included in the time dependent value for the angular distribution to position information and thus calculates the effective irradiation emittance. The effective irradiation emittance represents emittance of a virtual beam bundle, the virtual beam bundle being formed by summing portions of the charged particle beam which are incident into the region of the measurement target.