In a virtual reality (VR) environment, a wide field-of-view (FOV) image contributes to improve the immersive experience and situation awareness. The typical FOV of a head mounted display (HMD) is around 20 to 40 degrees, and widening this has been a technological challenge for decades. We designed a new HMD that has a hyperbolic half-silvered mirror. This HMD can resolve the technological challenges and provide a wide FOV, large observational pupil, and optical see-through display. We made a prototype HMD and a VR walkthrough system that displays a pair of stereo images with a FOV of over 120 degrees. This paper reports our HMD ' s basic concept and the implementation of the prototype.