"The global economy is not making use of great potential that is available. And that needs to change, not just for women's sake, but for economy's sake," Lagarde, head of the International Monetary Fund, said in a Tokyo conference on women in the labour market.
"What about Japan? Here, we have shown through our study that gradually raising the female labour force to the average level of the G7 could raise income per capita by four per cent -- permanently," she said.
"To target something even higher, which is the level of Northern Europe, then that would give an additional four per cent on the top," she said.
Gender gaps in labour force participation exist all over the world, and "Japan is somewhat in the middle" with a female participation rate of 65 per cent, 20 per centage points lower than for males, Lagarde noted.
"We know that the government is working flat-out to level the playing field," she said.
Abe has said Japan wants to boost the ratio of women in senior positions to 30 per cent by 2020, up from 11 per cent now, one of the lowest rates in the world.
In a recent reshuffle, he attempted to lead the way by appointing five women to his cabinet of 18 people.
"You have set ambitious targets Mr Prime Minister but I'm sure that they can be met. I know it's been disputed... some economists are arguing this is impossible," Lagarde said in the forum after it had been opened by Abe.
"As (US President) John Fitzgerald Kennedy once said, we don't go to the Moon because it's easy. We want to go because it's difficult, that's what you're doing," she said.
"We believe that these measures can actually pay off, adding a quarter per centage point to growth each year if implemented aggressively," Lagarde said.
"This is significant. Even so, it falls short of the one per cent boost to growth that Japan needs for Abenomics to succeed with flying colours.
"So while women can save Japan, they need help from other structural reforms," she continued.
The coveted reforms include boosting the availability of childcare services and carers for elderly parents in the world's fastest-ageing economy by loosening Tokyo's strict immigration rules, she said.
"One option is skilled immigrants," she said, adding "everybody would be happy" if Japan opens its borders more.
"Everybody would get the jobs they need. Everybody would get the care they need. And Japan could shape its own demographic destiny," she said.
Lagarde said Japan's culture of long working hours is also an obstacle for women, who frequently find it almost impossible to juggle childcare with the required after-work boozing that makes up the "exclusive 'boys' network".
AFP