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The structure of microvoids formed in the hydrogenated amorphous silicon (a-Si:H) layer of a-Si:H/crystalline silicon heterojunction solar cells has been considered as an important key to determine photovoltaic performance. However, due to limitations in accurately measuring the microvoid structure in the nanometer-thick region, the size and distribution of the microvoids near the heterointerface have not yet been clarified. Here we demonstrate a new method of optical characterization with nm-thick sensitivity to investigate the microvoid structure realized based on experimental data of positron annihilation spectroscopy and spectroscopic ellipsometry.