A treatment tool generator includes: a power source configured to supply power to a treatment tool; and a processor configured to control an operation of the power source and the treatment tool, the processor being configured to cause the power source to supply the power to the treatment tool to apply treatment energy to a biological tissue from the treatment tool, calculate a thickness index value to be an index of a thickness dimension of the biological tissue that is grasped by the treatment tool, determine whether it is a change timing to change a control state of at least one of the power source and the treatment tool based on the thickness index value, and perform at least one of a first control, a second control, and a third control upon determining that it is the change timing.