PROBLEM TO BE SOLVED: To provide an ion acceleration technology and a particle beam medical treatment device, capable of suppressing variation in current value for each bunch when circulating and accelerating an ion beam in a synchrotron.SOLUTION: An ion accelerator 10 comprises: a laser ion source 11 that generates a plasma by using a laser a linear accelerator 12 that extracts ions from the plasma and outputs an accelerated ion beam a synchrotron 16 that is provided with a deflection electromagnet 17 for circulating the ion beam on a circular track, and a high-frequency acceleration cavity 22 for accelerating the ion beam circulating on the circular track, and that receives the outputted ion beam to circulate and accelerate it a non-acceleration circulation time setting part 28 that sets a time for circulating the ion beam on the circular track without accelerating as a non-acceleration circulation time and a synchrotron controller 27 that activates the high-frequency acceleration cavity 22 to circulate and accelerate the ion beam after a lapse of the non-acceleration circulation time from when the ion beam enters into the synchrotron 16.
