A domestically-developed rocket launched with no glitches from a small island in southern Japan at 10:31 am (0131 GMT), carrying the country's hopes of restoring pride in its troubled space program.
The orbiter separated from the H-2A rocket about 45 minutes after it took off from the Space Center on the island of Tanegashima.
"We successfully launched the rocket and released the orbiter from the rocket," said Eriko Sunada, a spokeswoman for the Japan Aerospace Exploration Agency in Tokyo.
The lunar orbiter, aiming to collect data for research of the moon's origin and evolution, will travel around the Earth before moving into an orbit of the moon in early October, officials said.
The one-year mission, which is several years behind schedule due to technical mishaps, is said by the agency to be the largest lunar mission since the US Apollo program began in the 1960s.
The explorer was named "Kaguya" after a beautiful princess who charms many men before ascending to her home, the moon, in a popular Japanese folktale.
The 55-billion-yen ($478-million) probe will consist of a main unit, which will orbit 100 kilometers (60 miles) above the moon, and two small satellites.
It will gather data on the distribution of chemical elements and minerals, as well as on topographical and surface structures.
The mission aims to study the gravity field and environment of the moon while searching for hydrogen, which is required to for water.
Japan has been expanding its space operations, and has set a goal of sending an astronaut to the moon by 2020.
It faced an embarrassing failure in November 2003, when it had to destroy a rocket carrying a spy satellite 10 minutes after lift-off, because a booster failed to separate.
The setback came just a month after neighboring China became the third country to carry out a manned space mission. China is pressing ahead with a program that includes space walks and dockings.
With the lunar orbiter, Japan hopes to keep the country one step ahead of China and other regional rivals like India, which are also expected to launch similar probes in coming months.
"Japan aims to build a station on the surface of the moon in 2025, and so we need to understand the moon. We need to develop the fundamental technology," said Satoki Kurokawa, another spokesman for the agency.
China is expected to launch as early as this month its Chang'e 1 probe, to be followed by India's Chandrayaan 1, later this year. The US National Aeronautics and Space Administration is expected to send up its own Lunar Reconnaissance Orbiter in late 2008.
"China's and India's scope of observation is different from ours," Kurokawa said. "They do not plan to focus on gravitational attraction, surface layer, and magnetic pull - three areas I think Kaguya can excel in its uniqueness."
Some experts, however, are cautious about the prospects for the Japanese mission.
"I'm sure technical difficulties will pop up, which will be the first thing scientists will have to deal with," said Jun Nishimura, professor emeritus of space physics at Tokyo University.
"I doubt the mission will make a thorough research, as it plans, in one go," he said. "Technical sticking points will surface, and solving them will pave the way for improvements."
© 2007 Agence France-Presse