To improve accuracy in focusing.

A high-sensitive line sensor 16B acquires light reception data with high luminance that is obtained from light intensity of a light point image of an object 2 illuminated by means of a laser illumination part 11. A low-sensitive line sensor 16A acquires light reception data of low luminance that is obtained from the light intensity of the light point image of the object 2. A data processing part 17 normally selects the light reception data of high luminance from the high-sensitive line sensor 16B and if a laser light is saturated when the light reception data of high luminance is obtained, selects the light reception data of low luminance. A detection part 18 detects a focusing state of an image of the object 2 based on the light reception data selected by the data processing part 17 so that the focusing state of the image of the object 2 is detected without fail, thereby allowing accuracy in focusing to be improved. The present invention can apply to, for example, a position detector used for a measurement device that measures a cross-sectional shape of an object.