A speed-changing hydraulic oil pressure control apparatus for an automatic transmission of an automobile having a gear-type transmission connected to an output shaft of an engine through a torque converter, said apparatus comprising means for detecting output torque Tout of the transmission, means for detecting the input revolution number Ne of the torque converter, means for detecting the output revolution number Nt of the torque converter, means for estimating the input torque Tin of the transmission from said input revolution number and output revolution number, and means for detecting the transition state of the speed change from said input torque and said output torque for controlling hydraulic oil pressure of speed-changing elements in the transmission, wherein the speed change transition state-detecting means comprises means for computing the torque difference between the input torque Tin and output torque Tout of the transmission, and means for comparing the computed torque difference with a predetermined value and judging the phase as the torque phase when the computed value is smaller than the predetermined value or judging the phase as the inertia phase when the computed value is larger than the predetermined value.