It is legal for the unit to pay only social security and not medical insurance.

Legal analysis: if it is illegal, the employer may be required to pay medical insurance. Social security includes endowment insurance, basic medical insurance, industrial injury insurance, unemployment insurance and maternity insurance. According to the law, the unit should buy five insurances including medical insurance for employees. Therefore, it is illegal for the unit not to pay medical insurance, and it should be required to make up.

Legal basis: Article 23 of the Social Insurance Law of People's Republic of China (PRC), employees shall participate in the basic medical insurance for employees, and both employers and employees shall pay the basic medical insurance premium according to the provisions of the state. Individual industrial and commercial households without employees, part-time employees who have not participated in the basic medical insurance for employees and other flexible employees can participate in the basic medical insurance for employees, and individuals pay the basic medical insurance premium in accordance with state regulations.