Provided is an optical wavelength conversion element which has high optical wavelength conversion efficiency and therefore can be applied to weak light having an intensity similar to that of solar light and also has good long-term stability, and therefore can be used suitably in a solar cell, a photocatalyst, a photocatalyst-type hydrogen / oxygen generation device, a light up-conversion filter and the like. The optical wavelength conversion element is produced by dissolving and / or dispersing both an organic photosensitizing agent (A) and an organic luminescent molecule (B), i.e., a combination that exerts a triplet-triplet annihilation process, in an ionic liquid (C), and is homogeneous and transparent when observed with naked eyes. The ionic liquid (C) has such a property that, when the ionic liquid (C) is washed with superpure water in a volume that is 9 folds the volume of the ionic liquid (C), the water obtained after the washing has a pH value of 5 or more.