The $4d$ photoionization and subsequent ${N}_{4,5}{P}_{1}{P}_{1}$ Auger decay in free barium atoms have been investigated with special emphasis on the anomalous intensity ratio in the Auger electron spectrum. In order to understand the details of the spectrum, a series of ab initio calculations based on the multiconfiguration Dirac-Fock method have been performed. Theoretical predictions are compared with high-resolution electron spectra measured using synchrotron radiation. An interference effect, induced through electron correlation, was found to give rise to the anomaly of the intensities.