The findings of the analysis were presented at the ITB fair in Berlin on Thursday.
The institute has analysed the attitudes and behaviour of over 12,000 German tourists.
According to the number of trips and the amount of spending on holidays, Germans are among top three "tourist" nations, together with USA and China.
In 2014, the average spending of German 55 million holiday-makers was some EUR 1000 per trip that lasted five or more days.
Last year, slightly over two million German guests visited Croatia, and in 2015, the number of German visitors may increase by 3%.