A gear-shift instruction device that notifies a driver of a target gear in a transmission in which a gear can be switched in response to a manual gear shift operation, the gear-shift instruction device comprising: a target notification unit (S1 to S3) that is configured to notify the driver of the target gear so as to cause at least an up-shift or a down-shift to be performed relative to the current gear based on a driving condition of a vehicle in which the transmission is mounted; characterized by a detection unit (S4 to S8) that, in a case where a notification was made by the target notification unit, if a shift change is made to a gear different from the notified target gear, is configured to detect the shift change as an unintentional shift change, wherein the detection unit comprises: a driving condition determination unit that is configured to determine whether the vehicle speed or an engine speed is greater than or equal to a predetermined value; a gear shift operation determination unit that is configured to determine whether the current gear is lower than the target gear notified by the target notification unit; a rapid deceleration determination unit that is configured to determine whether a negative acceleration of the vehicle is greater than or equal to a predetermined value; and a processing unit that is configured to determine that a shift change is an unintentional shift change in a case where an affirmative determination was made by the driving condition determination unit, the gear shift operation determination unit determined that the current gear is lower than the target gear, and an affirmative determination was made by the rapid deceleration determination unit. a target notification unit (S1 to S3) that is configured to notify the driver of the target gear so as to cause at least an up-shift or a down-shift to be performed relative to the current gear based on a driving condition of a vehicle in which the transmission is mounted; characterized by a detection unit (S4 to S8) that, in a case where a notification was made by the target notification unit, if a shift change is made to a gear different from the notified target gear, is configured to detect the shift change as an unintentional shift change, wherein the detection unit comprises: a driving condition determination unit that is configured to determine whether the vehicle speed or an engine speed is greater than or equal to a predetermined value; a gear shift operation determination unit that is configured to determine whether the current gear is lower than the target gear notified by the target notification unit; a rapid deceleration determination unit that is configured to determine whether a negative acceleration of the vehicle is greater than or equal to a predetermined value; and a processing unit that is configured to determine that a shift change is an unintentional shift change in a case where an affirmative determination was made by the driving condition determination unit, the gear shift operation determination unit determined that the current gear is lower than the target gear, and an affirmative determination was made by the rapid deceleration determination unit. a driving condition determination unit that is configured to determine whether the vehicle speed or an engine speed is greater than or equal to a predetermined value; a gear shift operation determination unit that is configured to determine whether the current gear is lower than the target gear notified by the target notification unit; a rapid deceleration determination unit that is configured to determine whether a negative acceleration of the vehicle is greater than or equal to a predetermined value; and a processing unit that is configured to determine that a shift change is an unintentional shift change in a case where an affirmative determination was made by the driving condition determination unit, the gear shift operation determination unit determined that the current gear is lower than the target gear, and an affirmative determination was made by the rapid deceleration determination unit.