A photonic-crystal (PC) resonator has attracted a great deal of attention for the strong light-matter interaction. Many attempts have been made to achieve a high-quality factor of the PC resonator, but they always have accompanied increases of modal volumes. In this work, we propose a novel method to enhance modal confinement of the PC resonator without compromising the quality factor. In the proposed structure, a thin low-index slot layer is embedded in a two-dimensional PC for vertical confinement, which results in a remarkable mode volume reduction without a decrease of the quality factor. By optimizing the slot thickness, a quality factor to mode volume ratio, which is a figure of merit for an optical resonator, could be increased by 8 times.