"I have no worries at all," he told reporters. "Before I took this job, I strictly followed every law."
If found guilty, Somchai would be disqualified as an MP and could no longer serve as prime minister.
The allegation was brought to the election commission by Senator Ruangkrai Leekijwattana, who also filed the complaint that brought down Somchai's predecessor, Samak Sundaravej.
Samak was ousted on September 9, when Thailand's constitutional court ruled he had violated the constitution by accepting pay to host TV cooking shows while in office.
An election commission spokesman said a subcommittee would be appointed to investigate the allegation and will forward any findings to the court.
The latest probe comes amid a tense political crisis in Thailand, with anti-government protesters occupying the prime minister's office compound calling for Samak's resignation.
Several thousand protesters remain camped in the grounds of Bangkok's Government House, saying they are holding their ground to see if Somchai meets their approval.