The present paper investigates the effectiveness of ultrasound for promoting solidification as well as relaxing the supercooling of erythritol, as a phase change material, in a lab-scale heat storage unit. Temperature distributions along the axial and radial directions of the heat exchange pipe during the heat release experiments were measured to evaluate ultrasonic irradiation effect on the promotion of solidification. Simultaneously, the solidification behavior of erythritol around the heat exchange pipe was examined to study the mechanism of supercooling relaxation elaborately. The obtained experimental results with a non-ultrasonic irradiation condition indicated that there was a large temperature difference in the melt accompanied by supercooling for several tens of degrees. As verified by both DSC and XRD analyses, it was found that two types of solid phases of erythritol might be formed depending on crystallization conditions. On the other hand, a more uniform temperature profile within the melt was observed when ultrasound was irradiated. It was found that ultrasonic irradiation prevented supercooling by such appropriate features as characterized by acoustic streaming and the propagation of sound pressure, etc. It was considered that ultrasonic irradiation might be used as a method to initiate the nucleation process and promote dispersion of crystals throughout the melt, resulting in the suppression of supercooling.