Kawamura, 20, started the final round two shots behind overnight leader Park Sung-joon but fired a four-under-par 67 at the Ibaraki Country Club to edge the South Korean by one shot and claim the 150 million yen (£945,220) title.
Park dropped shots on the 16th and 17th but still had a chance to force a playoff at the co-sanctioned event but he misread his 10ft birdie putt on the par five to leave Kawamura celebrating.
"I am more surprised than anything as I was expecting a play-off," Kawamura was quoted as saying by the Kyodo news agency.
"This win is really big. Obviously it is the biggest thing to happen in my career. Now I get to play in Japan and on the Asian Tour as well. I'm sure it will brighten my future and help me improve," the Japanese told the Asian Tour.
Kawamura reached the turn one-over for his round but came to life on the back nine with five birdies and no drop shots to claim the breakthrough victory with a nine-under 275 total and moved him upto seventh on the Japan Golf Tour moneylist.
Park's sloppy finish meant he signed for a one-under 70 and eight-under total and his fourth runner-up finish on the Japan Golf Tour this season.
"Maybe next time I will have better luck. I'm still playing okay so that's the positive I can take back from this week," said Park, who is second on the Japan money list.
Another Korean to falter on the back nine in Osaka was former US PGA champion Yang Yong-eu, who was one back off Park after 10 holes but a poor tee shot at the 11th cost put paid to his title bid as he finished two off the pace.
"I played good. I only missed my drive on the 11th hole and dropped three shots there. My game is in great shape at the moment and I'm happy with where it is heading," Yang said.
China's Liang Wenchong shot a final round 68 to finish in a tie for fourth on 279 along with Australian Brad Kennedy (70) and Japan's Akio Sadakata (71).
($1 = 98.2550 Japanese yen)
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