Virtual reality (VR) and augmented reality (AR) with head-mounted displays (HMDs) are rapidly becoming popular. Unfortunately, optical see-through HMDs for AR are not compatible with VR and users must prepare the other type of HMD to experience VR. We present a new method for optical see-through HMDs that enables switching between VR and AR by changing the degree of transparency. The proposed method controls liquid crystals to change the transparency in an enclosure constructed of polarizing plates to achieve the switch. The proposed method does not require the use of expensive modules and is applicable to various HMDs including those that are smartphone based and retinal projection based. We construct a prototype implementing the proposed method and evaluate its performance and unique characteristics.