A Japanese high court quashed Friday a ruling that handed an 18-year jail term to a man responsible for a fatal road rage incident in 2017, sending the case back to a lower court due to an illegal procedural issue.

The Tokyo High Court said it agreed with the Yokohama District Court's conclusion that Kazuho Ishibashi, 27, caused an accident resulting in the death of a couple and injuries to their daughters on a highway. But the high court said the case should be reheard by a lay judge because the technical issue could have affected the lower court's ruling.

The decision disappointed the bereaved family as the high court revoked the lower court's ruling despite supporting its conclusion on the man's responsibility for the incident.

(Minivan rear-ended by truck on Tomei Expressway, killing couple and injuring daughters on June 5, 2017)

The district court convicted Ishibashi of dangerous driving after he overtook the car of Yoshihisa Hagiyama, 45, and his family on the Tomei Expressway in Kanagawa Prefecture, near Tokyo, on June 5, 2017, and forced it to stop in the passing lane.

While stopped, the car was struck from behind by a truck.

The lower court ruling in December last year said Ishibashi became enraged after being warned by Hagiyama about the way he parked his car at an expressway parking area just before the incident. Ishibashi pursued Hagiyama who was traveling with his wife, Yuka, 39, and their two daughters.

Presiding high court Judge Yoshifumi Asayama said the district court illegally overturned a view expressed at a pre-trial hearing that Ishibashi's act did not constitute dangerous driving under the law.

"The accused and his defense were caught off guard (by the change of the court's view). It was an illegal procedure that affected the ruling," Asayama said.

After being covered widely in the media, the case sparked public concern about dangerous driving in Japan, leading to a police crackdown and calls for traffic laws to be revised.

Hagiyama's mother told a press conference in Tokyo that she feels "frustrated" by the high court ruling as a jail term for Ishibashi remains undecided.

On the other hand, Ishibashi's lawyer Takashi Takano expressed hope for additional lower-court sessions, saying, "If we review evidence and fully discuss the accountability of the truck driver...there will be a good chance that the defendant's responsibility will be reduced."

(Mother of Hagiyama (center R) speaks to reporters in Tokyo after high court ruling)