Pakistan joined Oceania Cup champions Australia [ Images ] , Pan American champions Canada [ Images ], Olympic and World champions Germany [ Images ], Asia Cup champions South Korea, England [ Images ], The Netherlands, African champions South Africa [ Images ] and Spain in clinching a spot in the mega-event.
Two more spots will go to the winner of the second qualifying event, currently underway in Invercargill, New Zealand [ Images ].
The third qualifying event will start next week in Quilmes, Argentina.
India qualify as hosts of the February 28 - March 13 tournament in New Delhi [ Images ].
Veteran penalty-corner specialist Sohail Abbas scored twice in Pakistan's victory.
Pakistan went on the offensive from the start but Japanese goalkeeper Katsuya Takase thwarted their initial attempts.
After the aggressive opening, the Asian giants settled down and tried to take control with Waseem Ahmed making his presence felt in midfield.
Pakistan opened the scoring through Abdul Haseem Khan in the 12th minute after a decisive run by Shakeel Abbasi.
They maintained the pressure, but Japan did not give up either and it took a penalty-corner in the 24th minute by the ever-reliable Abbas to increase the lead.
Japan reduced the gap through a clever penalty-corner by their captain Kazuhiro Tsubouchi, who flicked the ball out of the reach of Salman Akbar in Pakistan's goal.
Pakistan earned a penalty-stroke early in second period but Abbasi put it wide and Japan nearly tied the game on their immediate counter-attack.
The Japanese players were closing down on the Pakistani attackers, preventing them from developing a fast game, but they could not do anything when faced with another stunning penalty-corner from Abbas that shaved the crossbar.
Even with a comfortable two-goal cushion, Pakistan were not out of the woods and survived two penalty-corners and a flurry of attacks by Japanese players.
In fact, the final 10 minutes were played in the defensive zone of Pakistan before the final buzzer assured them a place in the World Cup.