The Japanese government has ordered inspections of all motorway tunnels similar to the one which partially collapsed yesterday.
The number of people known to have died has risen to nine.
Photos from the interior of the tunnel suggest a possible weakness in its central column.
The collapse happened early on Sunday morning 80 kilometres from Tokyo on the motorway linking the capital to western Japan.
As part of the ceiling fell in, concrete panels crashed onto cars, trapping people and causing a fire.
The operating company says the tunnel, which is 4.7 kilometres long, is inspected five times a year. The panels had not been reinforced since being built in 1977 but passed safety checks last September.
It is thought that metal rods used to secure them may have come loose. An investigation has been ordered into possible negligence.
The government says significant investment will probably be needed to upgrade much of the country's infrastructure dating from the 1950s and 1960s.
Japan, earthquake-prone and covered in volcanic mountains, has thousands of road and rail tunnels.