Contrary to its highest R&D intensity, Japan has dramatically decreased its productivity in the 1990s. This can be attributed to a low marginal productivity of technology. Such a productivity decrease compels a delay in R&D, which results in reduced R&D productivity leading to a vicious cycle between R&D and its return. In this context optimal timing of R&D is crucial, particularly during economic stagnation and consequent difficulties in financing R&D. This paper analyzes the rationale for optimal timing of R&D and its interacting relationship with marginal productivity of technology. Empirical analyses are attempted focusing on Japan’s leading high‐technology firms and their innovative products.