Task instructions have traditionally been communicated orally in many fields. However, recently more and more wearable displays, such as the see-through head mounted displays (HMDs) have been developed, and some studies have provided ideas on applying visual instruction using these new interfaces to particular situations. However, in some cases, where instructions are communicated amongst the workers, the data is not sufficient for field workers to choose the best way of communicating instructions depending on the situation. Thus, this study aims to clarify the cases in which it is effective to apply visual instructions with HMDs, and to provide information that suggests the applicability of such visual instructions instead of or in addition to the traditional auditory instructions in different situations. These suggestions will be a useful reference for workers in safety-critical fields, helping them make better decisions about whether, when, and where to introduce the new method of instructions. It will also address some of the unsolved problems in the field, such as errors, low efficiency, and discomfort in communication.