The Japan Aerospace Exploration Agency says an infected computer was disconnected from its networks after a virus was found earlier this month. It is checking to see if any data was siphoned from it.
JAXA said in a statement that information about the Epsilon, due to be launched next year, its M-5 rocket and H2A and H2B rockets may have been compromised.
The agency said it is tightening security to prevent any further leaks.
Japan is hoping to compete with the U.S., Russia, Europe and others as a satellite launch vehicle provider and has developed the Epsilon to reduce costs and speed up launch times.