An explanation of how to trade Japanese Government Bonds (JGBs) is available at the link below.

The secondary bond market is split between exchange market trades and over-the-counter (OTC) transactions. 2-, 5-, 10-, 20-, and 30-year fixed rate JGBs are listed on the Tokyo, Osaka, and Nagoya stock exchanges, with transaction volumes published daily. Japan has five major exchanges for corporate equities, bonds, and derivatives. Each exchange follows its own listing requirements and trading system. For instance, the Tokyo Stock Exchange (TSE) uses its Tokyo Stock Exchange Trading Network System (ToSTNET), which also offers a platform for off-hours trading.

JGBs are predominantly traded OTC, where, in principle, price is determined through negotiation between the parties. The Japan Securities Dealers Association uses “fair business practice” regulations requiring each securities company follow rules to ensure fair and smooth OTC bond transactions. To improve price setting on the OTC market, the association publishes reference prices for OTC bond transactions each business day based on member reports and other financial institutions.

Bond futures are concentrated either medium-term government bond futures (5-year JGB), long-term government bond futures (10-year JGB), or super-long-term government bond futures (20-year JGB). JGB futures are both traded over-the-counter and at the Tokyo Financial Exchange (TFX) and the TSE.