Different factors that affect the beam emittance of highly charged ions extracted from the RIKEN 28-GHz superconducting ECR ion source have been investigated. Since the ion source parameters, space charge effects and various beamline components directly affect the beam quality, it was attempted to experimentally determine the different components that contribute to the beam emittance growth. From the experimental measurements, a beam emittance defined by the ion source has been evaluated based on a simple model calculation for space charge induced beam emittance. Ion beams using 40Ar11+ and 136Xe20+ ions were generated, and the beam emittance from the ion source and space charge were fitted from the experimental results. These parameters will be used to further evaluate the relationship between the extracted beam emittances and the ECR ion source parameters.