Lawyer Krisadang Nujaras said the court argued there was no reason to reverse the detention order.
Mr Krisadang also said three protest leaders - Promsorn Weerathamcharee, Sirichai Nathuang, and Mr Parit - are confirmed to have COVID-19.
The lawyer said he believed they contracted the virus while in detention as they tested negative before being detained.
He said prison officials have also turned down a request to transfer the trio to Thammasat University Hospital for treatment.
Meanwhile, the Corrections Department yesterday gave details on the health condition of the protest leaders.
Deputy department director-general Thawatchai Chaiwat said doctors at the Medical Correctional Hospital examined Mr Promsorn, Mr Parit and Mr Sirichai who were sent there for COVID-19 treatment.
According to doctors, Mr Parit had a headache, coughed phlegm, and body aches for three days.
However, his vital signs and the amount of oxygen in his blood were normal.
His breathing was normal and he was able to eat and sleep normally.
Mr Parit is categorised as a yellow-coded patient with no severe symptoms as he weighs over 90kg and has asthma as an underlying health condition.
Doctors have administered him with Favipiravir, asthma relief inhalers and other medication for symptomatic treatment, Mr Thawatchai said.
Mr Sirichai is categorised as a green-coded patient with mild symptoms, and he has been given Favipiravir, Mr Thawatchai said, adding the same diagnosis was given about Mr Promsorn.
All three have received close medical attention, he added.