A controller is configured to determine a current gear ratio step determined on the basis of gear ratio calculation Rg to be actual gear ratio of revolution Nm of main shaft 3a and revolution Nc of counter shaft 3b as well as gear ratio map for defining gear ratio variable allowance range for each of gear ratio steps. When making a decision of status of the determined current gear ratio step, the controller is configured to make delay timer get started when the gear ratio calculation Rg goes within the gear ratio variable allowance range of next step and halt to make a decision of the current gear ratio step while the delay timer is activated. The controller is also configured to make a decision of the status of the current gear ratio step after a predetermined delayed time Tn*(n=1, 2, . . . ) is elapsed.