I heard that japanese diet is the healthiest. Is this true?

No. People from okinawa are the longest living people in the world. Their diet is not the typical japanese diet.
? Reportedly okinawans have the longest lives. Okinawa was under the control of china, japan and the us at different times. Having lived there - some stick to a traditional japanese diet. Some have adopted a number of western foods as well as chinese foods. What is clear is that a diet high in vegetables and fish with little "junk food" can be beneficial.