Hun Sen, visiting Japan for his 15th time, met with business leaders and signed a bilateral agreement with his counterpart Shinzo Abe on encouraging investment.
"By strengthening good governance, we will continue to thoroughly manage public agencies and to eradicate corruption and reduce costs for investors," Hun Sen said at the luncheon with business leaders.
"I firmly believe that Japanese investment is essential for further development of Cambodia," Hun Sen said.
Hun Sen himself has been embroiled in accusations of poor governance.
Earlier this month, London-based environmental group Global Witness -- in a report banned by the Cambodian government -- accused the political elite including Hun Sen's relatives of illegally logging the nation's forests.
Japan is the top donor to Phnom Penh, but the world's second largest economy accounts for a mere two percent of Cambodia's overall trade, according to Japanese official data.
Japan offered Thursday fresh grant aid of 295 million yen (2.4 million dollars) for a project to improve drainage and flood protection in Phnom Penh.
Donors are meeting in Cambodia next week, but many have expressed deep frustration over the lack of reform in the country including corruption.
Hun Sen has frequently praised China, which has a growing rivalry for influence with Japan, for handing over aid without any conditions.
But he said here: "Aid from Japan and China are both very important. I don't want to skip over Japan and look to China, nor the other way around."
"Japan and China are both irreplaceable important players when talking about an idea of an East Asia community," he said. "I am glad that the bilateral ties between Japan and China are increasingly more friendly."
Hun Sen said Cambodia was on a steady growth track, overcoming the regional economic crisis in the 1990s and occasional natural disasters.
Recently discovered natural resources, such as oil and natural gas, should also boost Cambodia's international standing, Hun Sen said.
"Cambodia historically had had difficulties and struggles. But we have overcome the difficulties with our strong will to develop the nation under democracy," he said.
Hun Sen voiced optimism that growth, which has surged ahead at around 9.0 percent a year since 2000, would help eliminate widespread poverty in Cambodia.
"It is my sincere hope that well thought-out social and economic development will further reduce poverty in our nation," he said.
He thanked Japan's consistent financial and other aid for his nation, adding that more assistance is needed to improve infrastructure, human resources, and business and technological know-how.
Japan pledged more than 100 million dollars at last year's donor meeting, which netted Cambodia some 600 million dollars in aid.
Hun Sen, who arrived late Wednesday and leaves Saturday, also had an audience with Emperor Akihito.
He will travel to the southern region of Fukuoka to observe a project in farm development, a top priority for Cambodia.