We have developed a dichroic beam splitter for the Subaru AO188, which reflects optical light (0.4-0.9 μm) for wavefront sensing and transmits near-infrared light (0.93-5.2 μm) for science observations. The beam splitter is made of 145mm × 200mm calcium fluoride substrate coated by fluoride and metal chalcogen compound multilayer, which should be a best way to realize high transmittance over wide wavelength range in the near infrared. However, since typical fluoride soft coating is less resistant to the moisture in the air, the fluoride coating become damaged as we use on the AO188 optical bench which is placed in the room temperature condition. We have performed several accelerated endurance tests of the beam splitter under high-humidity condition by changing the design of the coatings, and found an optimal solution with an oxide protection layer which prevents the damage of the dichroic coating and keeps high transmittance at near-infrared wavelength. In this paper, we report the results of the endurance tests and the performance of our dichroic beam splitter.