An electronic circuit unit includes: a cable having a core of stranded thin wires, the core being covered with an outer surface, the outer surface at one end of the cable being removed to expose the core； a metallic tube that is tubular, the exposed core being inserted into an opening portion of the metallic tube； and a substrate including a connection land for connecting the core. The exposed core is connected to the connection land via the metallic tube by diffusion joining or ultrasound joining, the core has a diameter of 0.02 to 0.50 mm, the metallic tube has a thickness of 10 to 50 μm, and the metallic tube has an inner diameter that is larger than the diameter of the core by equal to or more than 1.1-fold and equal to or less than by 1.5-fold.
