1. An image capturing device comprising: an image capturing unit configured to receive light from an object and generate an image in which the object is captured; a continuously-captured-image synthesizing unit configured to generate a synthesized continuously-captured image in which the object is captured, by synthesizing two or more images temporally continuously captured by the image capturing unit; and a parameter setting unit configured to set a shutter speed at which each of the two or more images is captured, according to a luminance value of the object and a speed of the object, wherein the parameter setting unit is configured to set the shutter speed such that a degree of deterioration in resolution of the synthesized continuously-captured image caused by blur and a degree of deterioration in resolution of the synthesized continuously-captured image caused by noise are comparable to each other.