A treatment device includes: a heater including a first surface and a second surface, and a heating pattern； a treatment tool bonded to the second surface and configured to apply high-frequency energy and thermal energy to a living tissue； and a flexible substrate including a first electrical conductor configured to supply high-frequency power to the treatment tool, a second electrical conductor configured to supply power to the heating pattern, and a pair of insulating layers. The first electrical conductor includes a first bonding portion exposed from the pair of insulating layers and bonded to the treatment tool, the second electrical conductor includes a second bonding portion exposed from the pair of insulating layers and bonded to the heating pattern, the treatment tool includes a projection configured to project toward the flexible substrate and surround the heater, and the first bonding portion is bonded to the projection.
