To realize quick automatic focusing in a simple configuration.

This is an automatic focusing device which has an imaging optical system 1 to take the image of a measured object 7 through an objective lens 8 provided facing it, a drive mechanism 4 to move the imaging optical system 1 in the direction to the optical axis of the objective lens 8, and a controller 5 to control the drive mechanism 4 which carries out drive control of the above drive mechanism 4 based on the image of the measured object 7 taken by the imaging optical system 1. It has a projection optical system 2 to project three projection patterns focused in different positions in the direction of the optical axis of the objective lens 8 on the measuring plane for the measured object 7 through the objective lens 8. It compares the projected image sizes of the three projection patterns obtained by the controller 5 in the imaging optical system 1 and controls their positions to make their sizes satisfy the predetermined relations.

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