A microscope apparatus comprising: a light source emitting laser light, an objective lens, and a scanning optical system (1, 1A, 1B), the scanning optical system (1, 1A, 1B) comprising a scanner (11, 31) including a scanner mirror (12, 12A, 12B) that deflects the laser light from the light source while changing a deflection angle of the laser light; a light splitting unit that is capable of splitting-off the laser light from an optical path of the laser light deflected by the scanner (11, 31); an optical-path combining unit that is disposed between the light source and the scanner (11, 31) and that causes the laser light split-off by the light splitting unit to travel toward the scanner (11, 31); and a reentry relay optical system that has 1× relay magnifying power and that is disposed between the light splitting unit and the optical-path combining unit, the reentry relay optical system relaying the laser light split-off by the light splitting unit so as to cause the laser light to be incident again on the scanner (11, 31) at the same position as an incident position of the laser light from the light source via the optical-path combining unit, such that the laser light after a first deflection by the scanner is given an angle according to the deflection angle relative to an optical axis of the objective lens as a result of the first deflection by the scanner and the laser light split-off by the light splitting unit and relayed by the reentry relay optical system becomes incident on the same incident position on the scanner, which is set at a same swivel angle as that for the first deflection, at an incident angle equal to the first output angle from the scanner, whereby the laser light deflected again by the scanner is output therefrom while being given an angle twice as large as the angle according to the first deflection relative to the optical axis of the objective lens. a light source emitting laser light, an objective lens, and a scanning optical system (1, 1A, 1B), the scanning optical system (1, 1A, 1B) comprising a scanner (11, 31) including a scanner mirror (12, 12A, 12B) that deflects the laser light from the light source while changing a deflection angle of the laser light; a light splitting unit that is capable of splitting-off the laser light from an optical path of the laser light deflected by the scanner (11, 31); an optical-path combining unit that is disposed between the light source and the scanner (11, 31) and that causes the laser light split-off by the light splitting unit to travel toward the scanner (11, 31); and a reentry relay optical system that has 1× relay magnifying power and that is disposed between the light splitting unit and the optical-path combining unit, the reentry relay optical system relaying the laser light split-off by the light splitting unit so as to cause the laser light to be incident again on the scanner (11, 31) at the same position as an incident position of the laser light from the light source via the optical-path combining unit, such that the laser light after a first deflection by the scanner is given an angle according to the deflection angle relative to an optical axis of the objective lens as a result of the first deflection by the scanner and the laser light split-off by the light splitting unit and relayed by the reentry relay optical system becomes incident on the same incident position on the scanner, which is set at a same swivel angle as that for the first deflection, at an incident angle equal to the first output angle from the scanner, whereby the laser light deflected again by the scanner is output therefrom while being given an angle twice as large as the angle according to the first deflection relative to the optical axis of the objective lens.