A power supply apparatus (200) is for a treatment instrument (100) including a probe (122) having electrical conductivity which vibrates, a grasping member (134) that is opened and closed with respect to the probe and an electrode (136) provided in the grasping member. The power supply apparatus supplies high-frequency power between the probe and the electrode. The power supply apparatus includes a resistance acquisition circuit (214) which repeatedly acquires a resistance value of electrical resistance between the probe and the electrode, a condition determination circuit (216) which acquires the number of times the resistance value satisfies a predetermined condition while the probe is vibrating and power is supplied between the probe and the electrode, and a determination circuit (218) which determines whether or not the probe and the electrode are electrically short-circuited based on the number of times.
