A substrate for a medical device, a portion of which is brought into contact with or inserted into a subject. The substrate includes a patient circuit conductively connected to the portion that is configured to be brought into contact with or inserted into the subject, and a ground-side circuit configured to perform at least one of transmission of a signal, reception of a signal, and supply of electric power on the patient circuit. The ground-side circuit is grounded by a protective ground to ensure safety of a manipulator of the medical device. The substrate also includes an insulating layer between the patient circuit and the ground-side circuit providing insulation between the patient circuit and the ground-side circuit, and an isolated circuit provided apart from the patient circuit and the ground-side circuit on the insulating layer and having a different reference potential from the patient circuit and the ground-side circuit.