Named "Seem", the app was built on the insight that while 48% of cases of infertility the problem lies with the man, most Japanese men either automatically assume it is the woman's problem, or are ashamed to get tested.
The idea behind the app was to create a test that could be used in private. Using a magnifying glass slide, the app uses the mobile phone's camera to analyse sperm count and motility.
The agency claims that the kit is now Amazon Japan's top selling in the health category and that 33% of men who have used Seen seek medical treatment.
Seem also won a gold and a silver Lion in this category.
This year a gold, two silver and two bronze Mobile Lions went to UK agencies.
The gold and a silver Lion was won by B-Reel London for the Official Gorillaz App, created to promote the new Gorillaz album.
Saatchi & Saatchi London won a silver and a bronze for Direct Insurance's "Fleetlights", a prototype of smartphone-controlled flying torch drones that provide light in the darkest parts of the UK.
The remaining bronze lion was won by Iris Worldwide London for "Adidas Neo Snapchat - #myneolabel".
US agencies earned two gold Lions, 11 silver Lions and 8 bronze Lions.
The gold went to David Miami for the controversial "Google Home of the Whopper" for Burger King.
Update: B-Reel made a change to its Cannes entry on 21 June that credited the gold Mobile Lion to its London office instead of its New York office.