An arc path formation unit, comprising: a magnet frame having an inner space, and comprising a plurality of surfaces (511, 512, 513, 514) surrounding the inner space; and magnets coupled to the plurality of surfaces (511, 512, 513, 514) to form magnetic fields in the inner space, wherein the plurality of surfaces (511, 512, 513, 514) comprise: a first surface extending in one direction; a second surface disposed to face the first surface and extending in the one direction; and a third surface and a fourth surface extending from both end portions of the first surface and the second surface in the extending direction, respectively, at predetermined angles with the first surface and the second surface, and disposed to face each other, whereby the arc path formation unit is configured to produce a magnetic field in the inner space by a first magnet located at a middle of one of the first surface and the second surface; a second magnet disposed on one of the third surface and the fourth surface; and a third magnet located on another one of the first surface and the second surface; wherein a first facing surface (521b; 521a) of the first magnet facing the another one of the first surface and the second surface has a polarity different from a polarity of at least one of a second facing surface (522b; 522a) of the second magnet facing the another one of the fourth surface and the third surface and a third facing surface (523b; 523a) of the third magnet facing the one of the second surface and the first surface; characterised in that said third magnet is located to be biased to one side of said another one of the first surface and the second surface and wherein a shortest distance between the second magnet and the third magnet is longer than a shortest distance between the first magnet and the second magnet and a shortest distance between the first magnet and the third magnet. a magnet frame having an inner space, and comprising a plurality of surfaces (511, 512, 513, 514) surrounding the inner space; and magnets coupled to the plurality of surfaces (511, 512, 513, 514) to form magnetic fields in the inner space, wherein the plurality of surfaces (511, 512, 513, 514) comprise: a first surface extending in one direction; a second surface disposed to face the first surface and extending in the one direction; and a third surface and a fourth surface extending from both end portions of the first surface and the second surface in the extending direction, respectively, at predetermined angles with the first surface and the second surface, and disposed to face each other, whereby the arc path formation unit is configured to produce a magnetic field in the inner space by a first magnet located at a middle of one of the first surface and the second surface; a second magnet disposed on one of the third surface and the fourth surface; and a third magnet located on another one of the first surface and the second surface; wherein a first facing surface (521b; 521a) of the first magnet facing the another one of the first surface and the second surface has a polarity different from a polarity of at least one of a second facing surface (522b; 522a) of the second magnet facing the another one of the fourth surface and the third surface and a third facing surface (523b; 523a) of the third magnet facing the one of the second surface and the first surface; characterised in that said third magnet is located to be biased to one side of said another one of the first surface and the second surface and wherein a shortest distance between the second magnet and the third magnet is longer than a shortest distance between the first magnet and the second magnet and a shortest distance between the first magnet and the third magnet. a first surface extending in one direction; a second surface disposed to face the first surface and extending in the one direction; and a third surface and a fourth surface extending from both end portions of the first surface and the second surface in the extending direction, respectively, at predetermined angles with the first surface and the second surface, and disposed to face each other, whereby the arc path formation unit is configured to produce a magnetic field in the inner space by a first magnet located at a middle of one of the first surface and the second surface; a second magnet disposed on one of the third surface and the fourth surface; and a third magnet located on another one of the first surface and the second surface; wherein a first facing surface (521b; 521a) of the first magnet facing the another one of the first surface and the second surface has a polarity different from a polarity of at least one of a second facing surface (522b; 522a) of the second magnet facing the another one of the fourth surface and the third surface and a third facing surface (523b; 523a) of the third magnet facing the one of the second surface and the first surface; characterised in that said third magnet is located to be biased to one side of said another one of the first surface and the second surface and wherein a shortest distance between the second magnet and the third magnet is longer than a shortest distance between the first magnet and the second magnet and a shortest distance between the first magnet and the third magnet.