The proportion of Japanese men in their 20s who want to marry has slumped, with many citing their income not meeting women's expectations as a reason not to tie the knot, think tank research showed.

With a dip in the rate for women as well and it still being relatively rare for children to be born out of wedlock in Japan, the figures point to a potential obstacle for Prime Minister Shinzo Abe's key policy of boosting the nation's low birthrate.

The survey showed 38.7 percent of single Japanese men in their 20s who were polled said they wanted to marry as soon as possible or wanted to marry eventually, down from 67.1 percent three years ago.