The blast wave from the sleeper bomb blew out the windows of surrounding buildings and could be felt for several kilometres, said police and residents.
The accident, in which two of the wounded suffered serious injuries, shook an industrial area in the town of Euskirchen near the former capital Bonn in western Germany.
"During earth works an excavator hit a World War II bomb which exploded," a local police spokesperson told AFP.
"There was a huge blast wave. In the vicinity of the accident site and surrounding streets, home windows shattered and garage doors were pushed in."
The ground below many Germany cities still contains unexploded ordnance dropped by Allied and Soviet forces in the Second World War, but most is safely defused when found.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ