The quake, measuring 6.1 on the US Geophysics Institute, occurred at 7:58 am local time (22:58 GMT Sunday) near Osaka, at a depth of 15.4 km.
At least one fire broke out in a house in northern Osaka, television pictures showed.
Prime Minister Shinzo Abe hastened to stress that the government was in a state of mobilization and said: "The priority is to save people's lives." He ordered his team to "immediately collect information on potential damage."
The earthquake caused power outages in some areas, causing train traffic to stop at a busy time, affecting more than 170,000 homes. The NRC did not see anything abnormal in the stations.