The case where a plurality of non-writing lines determined to have the correlation with both writing lines 22a1 and 22a2 exist will be described below. In this case, when determining that the plurality of non-writing lines have the correlation with both writing lines 22a1 and 22a2, determination unit 32 further performs processing of narrowing writing lines 22a1 and 22a2 having the correlation with the plurality of non-writing lines to one. That is, determination unit 32 associates one writing line with the plurality of non-writing lines. An example in which non-writing lines 22b23 and 22b25 are determined to have the correlation with both writing lines 22a1 and 22a2 will be described below.