As technologies' life-cycle shortens and their development directions are uncertain, firms' technology planning capability becomes increasingly important. Prior patent-based studies using technology growth curves identify developmental stages of technologies, thereby formulating technology development directions from an overall perspective. However, a technology generally consists of multiple sub-technologies and accordingly their development stages are likely various. In this regard, the prior studies failed to identify core sub-technologies and their specific development directions. Therefore, we suggest an approach consisting of 1) identifying core sub-technologies of a given technology using patent co-classifications and social network analysis, and 2) identifying each sub-technology's development stage and thereby determining its further development direction. We apply our approach to patents related to augmented reality to examine its applicability. It is expected that our approach will help identify evolving development stages for the core sub-technologies of a given technology, thereby effectively assisting technology experts in technology planning processes.