The carbon stock function of harvested wood products (HWPs) is attracting attention among climate change countermeasures. Among HWPs, particle board (PB) and fiberboard (FB) mainly use recycled materials. This study estimated carbon stocks of PB and FB and their annual changes over the past 70 years in Japan using three methods of the Intergovernmental Panel on Climate Change guidelines: Tiers 1–3. Tier 1 uses first order decay (FOD), a 25-year half-life, and the Food and Agriculture Organization of the United Nations database. Tier 2 uses FOD, a 25-year half-life, and Japan-specific statistics. Tier 3 uses a log-normal distribution for the decay function and a 38–63-year half-life of building PB/FB. Japan’s PB and FB carbon stocks have increased for the past 70 years. The latest carbon stock in early 2022 and the annual change in carbon stock in 2021 was 21.83 million t-C and 0.42 million t-C/year, respectively for Tier 3. Tier 3 has the highest estimation accuracy by using decay functions and half-lives that match the actual conditions of building PB and FB, whereas Tiers 1 and 2 were underestimates. Approximately 40% of the carbon stock is derived from waste wood, which extends its utilization.