China bought a net 1.33 trillion yen ($1.66 billion) in Japan long-term debt in April, which is the largest amount since January 2005, according to the data released by Japan's Ministry of Finance, Bloomberg reported Wednesday.

"As China tries to diversify its assets with its huge foreign-exchange reserves, it probably wants to have yen-denominated assets to some extent," Tetsuya Inoue, chief researcher for financial markets for Tokyo-based Nomura Research Institute Ltd, was quoted by Bloomberg.

Japanese long-term debt brought investors a 2.2 percent gain since April, versus an average 1 percent return rate for the broad market, based on Bank of America Merrill Lynch data. The Nikkei 225 Stock Average has fallen 3.1 percent over the same period, the Bloomberg report said.