Uichiro Niwa, the newly retired Japanese ambassador to China, left Beijing quietly on Wednesday morning as questions remain over strained Sino-Japanese ties.

Niwa, 73, witnessed ups and downs in the two countries' relations over the past two years and four months, with the most turbulent moments coming during the territorial dispute over the Diaoyu Islands in the East China Sea.

Tokyo ran into a major diplomatic standoff with Beijing after the Japanese government illegally "purchased" part of the islands in mid-September.

During a news conference in Beijing on Monday, Niwa said his mission in China "both started and ended with" the islands dispute.

Niwa, an experienced entrepreneur, was regarded as a breath of fresh air in Japanese diplomacy when he took over as Tokyo's top envoy to Beijing in July 2010.

However, several months after he took office in Beijing, Niwa encountered a major diplomatic crisis after a Chinese fishing trawler collided with two Japanese coast guard vessels in the waters off the Diaoyu Islands.

Bilateral ties warmed in 2011 when a Chinese search and rescue team was sent to Japan after the March 11 earthquake and tsunami. Premier Wen Jiabao also visited the country.

This year marks the 40th anniversary of the normalization of diplomatic relations between the two countries in 1972, and Niwa had mapped out plans for a series of commemorative events.

Tensions again flared up in April when former Tokyo governor Shintaro Ishihara announced his plan to "purchase" the Diaoyu Islands. Ishihara even kicked off a public fundraising campaign.

The ambassador was dragged into the controversy after giving an interview to the Financial Times. When he was asked about Ishihara's plan, he warned about the potential for a grave crisis in bilateral ties.

Niwa was blasted from within Japan for his differing views and remarks, which strayed from Tokyo's hard-line stance.

"Japan's strong nationalist mood cannot tolerate a second place behind China and will not give up its goal of becoming a political power in the world. Therefore Japan's policies toward China show a lack of coordination and sometimes even self-contradiction," said Yang Bojiang, a professor of Japanese studies at the University of International Relations.

Niwa kept a low profile after the interview. In July, Japanese Prime Minister Yoshihiko Noda announced a plan to "nationalize" the islands, a move that further heated bilateral tensions, and Niwa was temporarily summoned back to Tokyo.

His successor, Shinichi Nishimiya, died days after being appointed in mid-September.

On Nov 15, the Japanese Cabinet approved the appointment of Masato Kitera, assistant chief cabinet secretary, as Japan's new ambassador to China. The appointment took effect on Monday.

Kitera, who is expected to take up his post within 40 days, is a veteran diplomat who is regarded as a typical bureaucrat of the foreign ministry. Observers are skeptical that his appointment will improve the frayed bilateral ties.

"It is of no use to change ambassadors, if Japan refuses to make a major change in its diplomatic policy," said Zhou Yongsheng, a professor of Japanese studies at China Foreign Affairs University.

Contact the writer at zhangyunbi@chinadaily.com.cn