Provided is a highly-reliable cooling structure for a power conversion device for rail cars which uses a self-oscillating heat pipe for cooling, wherein a decrease in cooling performance due to impact by a flying object or the like is avoided. Accordingly, this power conversion device for rail cars is composed of a plurality of power semiconductor elements and comprises a self-oscillating heat pipe which cools the power semiconductor elements. The self-oscillating heat pipe is, for example, structured such that the interior of a heat sink member exposed to a cooling airflow has sealed flow paths wherein a working fluid is sealed. The thickness of end partition walls of the self-oscillating heat pipe, each of which is in contact with an airflow inlet portion or an outlet portion, is greater than the thickness of an internal partition wall separating the flow paths.