LACONIA, N.H. (AP) _ A grand jury blamed the state for public ignorance of child-restraint laws in refusing to indict a man whose 8-year-old son was not wearing a seat belt when he was killed in a traffic accident.

The grand jury in Belknap County said Toshio Minagawa, 42, of Andover, Mass., behaved plausibly when he failed to buckle up his son, Kohei, who was thrown from the family van in a Dec. 29 crash on Interstate 93 in Sanbornton in eastern New Hampshire. Minagawa's daughter, Yoko, 12, was seriously injured.

Minagawa would have been the first person indicted with negligent homicide under a state law that requires auto passengers up to age 12 to be buckled in.

The jury said Thursday that it might have reached a different conclusion if New Hampshire ''better informed its motorists of the mandatory requirement.''

''We strongly urge the state to erect signs along its highways advising of this requirement,'' the jury said in a statement. ''Perhaps by so doing such tragedies may be averted.''

It said that although not belting a child is illegal, Minagawa's action was not ''a gross deviation from the conduct a reasonable person would observe.''

Earlier, County Attorney Ed Fitzgerald said he hoped publicity about the threat of a grand jury indictment would make the public to take the child- restraint law seriously.

''We want people to sit up and take notice that ... but for the lack of a seat belt, the child would be alive today,'' he said.

A similar negligent homicide case in Strafford County in which a 16-month- old boy was killed was rejected by a grand jury in 1989.