The costs of 167 public works projects completed between fiscal years 1993 and 2002 were between 50 percent and 1,000 percent more than the government had estimated, according to a report released Friday.
The government said time-consuming negotiations with landowners were the main reason for the ballooning costs.
In many cases, commodity and land prices rose substantially while the negotiations continued.
According to the report, the Land, Infrastructure and Transport Ministry undertook 104 of the projects and the Agriculture, Forestry and Fisheries Ministry carried out the remainder.
The report covered public works projects that involved a minimum 10 billion yen in expenses.
Spending for the projects also increased due to changes in construction methods that were found to be necessary in studies conducted after construction work had begun, it said.
Critics say the government estimates lower figures for project costs to obtain public support. The costs then blow out due to government refusal to cancel the projects after they have begun -- even when the projects become unnecessary.
Of the 167 projects, the one whose costs increased the most was a farmland development plan in Fukushima Prefecture, which began in December 1968.
Its initial budget was 4.7 billion yen, but by the time it had been completed in 1998, it had cost 56.1 billion yen. The more than 1,000 percent increase was due to price increases during and changes in construction methods.