The present invention discloses a device for acquiring a functional image of tissue and a method for acquiring a functional image by using same, the device comprising: a light source for irradiating a tissue to be imaged with coherent light; an image acquisition unit for acquiring an image of a speckle pattern which is formed by scattering the light emitted from the light source over the tissue, and acquiring multiple images having different exposure times; an image processing unit for generating a functional image of the tissue on the basis of the multiple images acquired by the image acquisition unit; and a control unit for adjusting the light quantity of the light emitted to the tissue such that the multiple images having different exposure times have brightness values in a common range, and controlling the operation of the image acquisition unit.