The nation's space authorities released a report Monday calling for a change in the design of the H-IIA rocket's booster nozzles, saying a hole could form as happened in the failed H-IIA launch in November.
A research panel of 14 experts on the government's Space Activities Commission concluded in the report that such a hole caused the failure of the last H-IIA launch.
Officials hope that pinpointing the cause of the failure will pave the way for the Japan Aerospace Exploration Agency (JAXA) to resume launches by the end of this year.
The hole in the nozzle's outer wall, which is made of reinforced carbon fiber, caused a leak of fuel that damaged a wire that was to separate the booster from the H-IIA rocket, it said.
The nozzle of the H-IIA became susceptible to such damage because the combustion pressure was doubled from that of the H-II predecessor rocket to save costs, the report says.
The nozzle accelerates exhaust gases from the combustion chamber to a high velocity.
Although damage to the booster nozzles was observed during test production in the development of the H-IIA rocket, measures to deal with the problem were not sufficient, the report says.
The domestically developed H-IIA is central to Japan's space program.
JAXA launched the sixth H-IIA on Nov. 29 from the Tanegashima Space Center in Kagoshima Prefecture to send two spy satellites into space.
The mission failed when the ground crew deliberately destroyed the launch vehicle shortly after liftoff because one of the two boosters failed to disengage after burning its fuel.
The first five H-IIA launches were successful.
Tsukasa Mito, director in charge of rocket launches at JAXA, said the agency plans to do its best for the next launch by fully reflecting the panel's report in technical improvements of the rocket's components.