There is provided a transmission control device capable to switching a switching mechanism quickly. A transmission control device ECU controls an automatic transmission (3) and can receive information about a slope from a front and rear G sensor. The automatic transmission (3) includes a two-way clutch (F1) as a switching mechanism that is switchable between a reverse rotation prevention state in which forward rotation of a rotatable carrier (Cb) (eleventh element) is allowed and reverse rotation is prevented and a fixed state in which rotation of the element is prevented. When the range is switched to a parking range and a vehicle is not stopped on a slope, the transmission control device ECU switches the two-way clutch (F1) to the fixed state.