An observation device 1 comprises a light source unit 10, a biaxial scanning system 20, a wavefront modulation unit 30, an optical branching unit 40, a light detection unit 50, a wavefront detection unit 60, a control unit 70, and the like. The wavefront modulation unit 30 presents a compensating phase pattern for compensating for an aberration of input light and a branching phase pattern for splitting the input light into first and second beams. The wavefront detection unit 60 receives inputted light and detects a wavefront of the inputted light. The compensating phase pattern for compensating for the wavefront aberration is feedback-controlled in loop processing that includes the detection of a wavefront distortion of the light by the wavefront detection unit 60, the adjustment of the phase pattern by the control unit 70 according to the result of detection, and the presentation of the phase pattern by the wavefront modulation unit 30.