PURPOSE: To eliminate the production of a non-conforming article out of a product specification of JAS by finding an effective length of a short-length veneer in a fiber direction and a standard length of a continuous-length longitudinally-joined veneer by a calculation in accordance with a formula.

CONSTITUTION: An effective length W of a short-length veneer in a fiber direction and a standard length L of a continuous-length longitudinally-joined veneer are found by a calculation in accordance with a formula. In the formula, (n) is a positive integer except 0, and P is a positive integer more than 0. A product posture of a continuous-length longitudinally-joined veneer is produced correspondingly to a standard-length L using a short-length veneer having an effective length W in a fiber direction, which meets a requirement that a longitudinally joined part should exist out of ranges of (W±10T)/6, (W±10T)/5, (W±10T)/4, (W±10T)/3, (2W±T10)/5, (W±T10)/2, (3W±10T)/5, (2W±10T)/3, (3W±10T)/4, (4W±10T)/5, (5W±10T)/6, and 30T<W/P<W-30T with respect to W/P where T is a veneer thickness.