The Japanese automobile industry reacted cautiously today to the Reagan administration's plan to aid the ailing U.S. auto industry but indicated it thought the medicine may not be strong enough.

Toyota Motor Sales President Sadazo Yamamoto said the package represented a "cool judgment" by the president and welcomed what he interpreted as Reagan's assessment that the industry's problem was domestic.

But Yamamoto said Japan had believed that Reagan would announce some measures to restrain U.S. auto workers' wages and to provide more investment incentives. "It seems that these have been put off," he said.

Japan is under pressure from the United States to restrain its auto exports to the United States voluntarily or face tough restrictive quotas that may be enacted by Congress. There have been indications that the government here will be willing to impose restraints, but it first has asked the Reagan administration to take positive action to help the American industry get back on its feet. It also has sought U.S. recognition that the American industry's main problem is one of its own making, not one simply caused by Japanese exports.

An auto industry source termed the president's statement in Washington "a little bit disappointing," because it did not express a strong demand for wage restraint and did not mention any special tax incentives to encourage the American industry's investments.

He welcomed the president's stress on free trade, but indicated he thought it was inconsistent to speak of free trade on the one hand and talk of Japan exercising voluntary restraints on the other.

There was no immediate definitive reaction from the Japanese government, which today opened discussions with a team of Reagan administration representatives on the issue.

Industry representatives said they would now wait to get their government's reaction before making any decisions about restraining exports. They indicated that they would like their government's negotiators to press the Reagan administration for more specific and substantive proposals.