A three-level power converting apparatus in which each of phases constituting a power converting portion is configured by a single unit, each unit comprising: four switching devices (1u-4u, 1v-4v) including first to fourth switching devices which are connected in series in this order between a positive terminal and a negative terminal of DC side; a first diode (9u, 9v) connected between a joint (7u, 7v) between said first switching device (1u, 1v) and said second switching device (2u, 2v) and a DC side middle terminal; a second diode (10u, 10v) connected between a joint (7u, 7v) between said third switching device (3u, 3v) and said fourth switching device (4u, 4v) and said middle terminal; and an AC terminal (14u, 14v) between said second switching device (2u, 2v) and said third switching device (3u, 3v); wherein said plurality of switching devices (1u-4u, 1v-4v) and said plurality of diodes (9u, 10u), (9v, 10v) of said units are arranged on a heat sink disposed on a common plane of a cooling device and said cooling device is cooled by a coolant flowing in one direction, each of said units being characterized in that said plurality of diodes (9u, 10u), (9v, 10v) are arranged adjacent to one another in a central area of said heat sink, long sides of said individual switching devices (1u-4u, 1v-4v) are oriented perpendicular to a flow direction of said coolant, said first switching device (1u, 1v) and said second switching device (2u, 2v) are arranged on one side of said plurality of diodes (9u, 10u), (9v, 10v) whereas said third switching device (3u, 3v) and said fourth switching device (4u, 4v) are arranged on the other side of said plurality of diodes (9u, 10u), (9v, 10v), and said plurality of switching devices (1u-4u, 1v-4v) are arranged along the flow direction of said coolant with said plurality of diodes (9u, 10u), (9v, 10v) sandwiched in between. four switching devices (1u-4u, 1v-4v) including first to fourth switching devices which are connected in series in this order between a positive terminal and a negative terminal of DC side; a first diode (9u, 9v) connected between a joint (7u, 7v) between said first switching device (1u, 1v) and said second switching device (2u, 2v) and a DC side middle terminal; a second diode (10u, 10v) connected between a joint (7u, 7v) between said third switching device (3u, 3v) and said fourth switching device (4u, 4v) and said middle terminal; and an AC terminal (14u, 14v) between said second switching device (2u, 2v) and said third switching device (3u, 3v); wherein said plurality of switching devices (1u-4u, 1v-4v) and said plurality of diodes (9u, 10u), (9v, 10v) of said units are arranged on a heat sink disposed on a common plane of a cooling device and said cooling device is cooled by a coolant flowing in one direction, each of said units being characterized in that said plurality of diodes (9u, 10u), (9v, 10v) are arranged adjacent to one another in a central area of said heat sink, long sides of said individual switching devices (1u-4u, 1v-4v) are oriented perpendicular to a flow direction of said coolant, said first switching device (1u, 1v) and said second switching device (2u, 2v) are arranged on one side of said plurality of diodes (9u, 10u), (9v, 10v) whereas said third switching device (3u, 3v) and said fourth switching device (4u, 4v) are arranged on the other side of said plurality of diodes (9u, 10u), (9v, 10v), and said plurality of switching devices (1u-4u, 1v-4v) are arranged along the flow direction of said coolant with said plurality of diodes (9u, 10u), (9v, 10v) sandwiched in between.