Judge Donald Stephens asked for the inquiry after he was alerted to the issue in a letter from Wake County District Attorney Colon Willoughby.
In his letter, Willoughby said he found "a disturbing pattern of what may be unauthorized and illegal judgments."
The DA said in at least 12 cases, a person was convicted of DWI in District Court and then the case was appealed to Superior Court. After a period of time, the appeals were withdrawn, and the cases were sent back to District Court for resentencing.
In all 12 cases, Willoughby said Judge Ruth - who was not involved in either the original sentencing or the resentencing - entered what appear to be orders that change the effective date of the sentencing without notifying prosecutors.
Willoughby said the orders effectively backdate the convictions and would shorten or eliminate the mandatory suspensions of the defendant's driver's licenses.
"In at least three of these cases, it appears that Judge Ruth has issued limited driving privileges for defendant without legal authority to do so," Willoughby wrote.
In his response to Willoughby's letter, Judge Stephens said there may be an innocent explanation but "from the face of the court files documented, I can think of none."
"This type of irregularity brings into question the integrity of the court system," he continued.
Stephens said while the SBI looks into the matter, he has asked that Judge Ruth not be assigned any criminal court cases.
Ruth was first elected to the bench in 1998. She has not returned a phone call for comment.