A transmission device (1, 1A, 1F) comprising: an input disc and an output disc which are disposed to face each other; a power roller which is tiltably disposed between the input disc and the output disc and transmits a rotational driving force of the input disc to the output disc in a transmission ratio corresponding to a tilt motion angle of the power roller; a trunnion on which the power roller is rotatably supported, a position of the trunnion being changed to change the tilt motion angle of the power roller; a trunnion drive mechanism which includes a speed reduction chamber and a speed increase chamber, changes the position of the trunnion to increase the transmission ratio in a case where the hydraulic oil is supplied to the speed reduction chamber and the hydraulic oil is discharged from the speed increase chamber, and changes the position of the trunnion to reduce the transmission ratio in a case where the hydraulic oil is supplied to the speed increase chamber and the hydraulic oil is discharged from the speed reduction chamber; a hydraulic pump which discharges the hydraulic oil to be supplied to the trunnion drive mechanism, and a direction selector valve which selects a flow direction of the hydraulic oil discharged from the hydraulic pump to supply the hydraulic oil to one of the speed reduction chamber and the speed increase chamber and discharge the hydraulic oil from the other of the speed reduction chamber and the speed increase chamber; characterised by: a discharge valve (65, 65A, 65D) which discharges the hydraulic oil from the speed increase chamber; and a controller which controls a movement of the discharge valve (65, 65A) to discharge the hydraulic oil from the speed increase chamber so that the transmission ratio reaches a maximum speed reduction ratio, in a case where a predetermined discharge condition is met. an input disc and an output disc which are disposed to face each other; a power roller which is tiltably disposed between the input disc and the output disc and transmits a rotational driving force of the input disc to the output disc in a transmission ratio corresponding to a tilt motion angle of the power roller; a trunnion on which the power roller is rotatably supported, a position of the trunnion being changed to change the tilt motion angle of the power roller; a trunnion drive mechanism which includes a speed reduction chamber and a speed increase chamber, changes the position of the trunnion to increase the transmission ratio in a case where the hydraulic oil is supplied to the speed reduction chamber and the hydraulic oil is discharged from the speed increase chamber, and changes the position of the trunnion to reduce the transmission ratio in a case where the hydraulic oil is supplied to the speed increase chamber and the hydraulic oil is discharged from the speed reduction chamber; a hydraulic pump which discharges the hydraulic oil to be supplied to the trunnion drive mechanism, and a direction selector valve which selects a flow direction of the hydraulic oil discharged from the hydraulic pump to supply the hydraulic oil to one of the speed reduction chamber and the speed increase chamber and discharge the hydraulic oil from the other of the speed reduction chamber and the speed increase chamber; characterised by: a discharge valve (65, 65A, 65D) which discharges the hydraulic oil from the speed increase chamber; and a controller which controls a movement of the discharge valve (65, 65A) to discharge the hydraulic oil from the speed increase chamber so that the transmission ratio reaches a maximum speed reduction ratio, in a case where a predetermined discharge condition is met. a discharge valve (65, 65A, 65D) which discharges the hydraulic oil from the speed increase chamber; and a controller which controls a movement of the discharge valve (65, 65A) to discharge the hydraulic oil from the speed increase chamber so that the transmission ratio reaches a maximum speed reduction ratio, in a case where a predetermined discharge condition is met.