1. An apparatus comprising: a device for photographing an end of a wood block and generating an image composed of pixels and representing the wood block end with annual rings thereof; a device for setting a plurality of seek starting points in the image of the wood block end; a filter having a center and a plurality of parallel straight lines spaced at a distance that is smaller than a spaced distance between any two adjacent annual rings in the wood block end image and rotatable about the center thereof to a plurality of different angular positions thereof; a device for setting the filter in the image of the wood block end, calculating a value representing a degree of spread-out of distribution of differences of pixel values of at least selected pixels lying on each of the straight lines of the filter from mean of said pixel values at each of said plurality of angular positions of the filter, calculating a sum of said values each representing the degree of spread-out of distribution for said plurality of straight lines of the filter at each of the angular positions of the filter, comparing the calculated sums and determining a line that is normal to the straight lines of the filter at an angular position thereof where the sum is the least as the normal line with respect to a line that is tangential to the annual ring, thereby generating such normal line, moving the filter for a predetermined distance along the generated normal line radially inwardly of the wood block end image, repeating said generating a normal line and said moving the filter for each of the seek starting points, thereby seeking and generating a plurality of lines of path of the filter starting from the respective seek starting points, and a device for specifying the center of annual rings in the wood block end image based on points of intersections of said lines of path of the filter.