To provide a substrate for a medical device and a medical device in which the reference potential of a patient circuit can be stabilized, the noise between the patient circuit and a ground-side circuit can be reduced, and the insulation distance between the patient circuit and the ground-side circuit can be sufficiently ensured without using a large-sized electronic component. A patient circuit that is a circuit on the side of being brought into contact with or inserted into a subject, a ground-side circuit that is a circuit provided on the side of performing processing on a signal transmitted from the side of being brought into contact with or inserted into the subject and protectively grounded, an insulating layer provided between the patient circuit and the ground-side circuit and 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 surface of the insulating layer and having a different reference potential from the patient circuit and the ground-side circuit are included.
