I have to quarry the reasons behind the similarity of the flag between Japan and Bangladesh. My thinking was maybe they got some political or any geographical relationship but I was not almost wrong they got some indirect similarities in meaning. First, the red circle in the flag of Bangladesh indicates the sun rising over Bengal, and the new Sun of independence which rose after she obtained independence from Pakistan.
Red symbolizes the sacrifices made by millions of people during the liberation war. As far as Japan’s flag goes, the red circle is the rising sun. Japan is the first country to view sunrise and they are known as the Land of Rising Sun. In both cases the circles are indicating “Dawn”. In the case of Bangladesh, it is the “Dawn” of new era of freedom.
In the case of Japan, it is a brand new day. The green field stands for the lushness of the land of Bangladesh and Japan's name translates to "The land of the rising sun." The white represents honesty and purity. As they got the similarity to their flag the same way these two country co-operating each other.