According to reports, he died on Sunday afternoon. On his way to the office, he felt sick and was first transferred to the hospital in Teplice and later died.
Kubera, 72, gave a speech at his party's congress over the weekend and was not known to be serious ill.
Last week, Kubera, along with the chairman of the Chamber of Deputies, Radek Vondracek, attended a President's New Year's lunch.
Since 2000 he has been a Senator for District No. 32 - Teplice - and the Chairman of the Committee on Constitutional and Legal Affairs of the Czech Parliament's Senate. Since 2016 he has been the Vice-Chairman of the Senate.
He was married and has two adult daughters.