Shares in Japan's Nintendo Co Ltd (7974.T) soared again on Monday, bringing market-value gains to $7.5 billion in just two days as investors cheered the runaway success of Pokemon GO - its first long-awaited venture in mobile gaming. The game, which marries a classic 20-year old franchise with augmented reality, allows players to walk around real-life neighborhoods while seeking virtual Pokemon game characters on their smartphone screens - a scavenger hunt that has earned enthusiastic early reviews. In the United States, by July 8 - two days after its release - it was installed on more than 5 percent of Android devices in the country, according to web analytics firm SimilarWeb.

It is now on more Android phones than dating app Tinder and its rate of daily active users was neck and neck with social network Twitter, the analytics firm said. The game is also being played an average of 43 minutes a day, more time spent than on WhatsApp or Instagram, it added.

As the game took the U.S. by storm, Nintendo's shares surged by a quarter in value on Monday to their highest level since November. They have gained 36 percent since Thursday's close with the initial momentum coming from the game shooting to the top of free app rankings in Apple Inc's (AAPL.O) U.S. iTunes store.

The game has been released in the United States, Australia and New Zealand. Launches for other countries including Japan - one of the world's biggest gaming markets - are due soon.

Pokemon GO may not prove an immediate boost to Nintendo's bottom line: it is free and Nintendo is not the sole investor or creator.

The game itself was created by Niantic, spun off from Google last year, and Pokemon Company. Nintendo owns a third of Pokemon Company and both have undisclosed stakes in Niantic, which had already developed a similar augmented reality, multi-player game in 2012.

Some analysts have been upbeat about the money-making potential for Pokemon GO, largely from small purchases made while playing - and the positive signs for other mobile gaming launches planned by Nintendo for 2016 and 2017.