A control device for a belt type continuously variable transmission, comprising a primary pulley for receiving an input from a drive source, a secondary pulley for providing an output to a drive wheel, and a belt wrapped around the primary pulley and the secondary pulley, the device comprising: a secondary hydraulic pressure control means configured to control a secondary hydraulic pressure to the secondary pulley on the basis of a command secondary hydraulic pressure to the secondary pulley, and characterized by a belt slip control means configured to estimate a belt slip condition by monitoring a phase difference between an oscillation component included in the actual secondary hydraulic pressure and an oscillation component included in the actual gear ratio, wherein the control device is configured to switch between a normal control and a belt slip control over the second hydraulic pressure, based on a belt slip control permission condition, wherein during the normal control the secondary hydraulic pressure control means determines a command secondary hydraulic pressure by a feedback control based on a deviation between a target secondary hydraulic pressure and an actual secondary hydraulic pressure, and wherein during the belt slip control the secondary hydraulic pressure control means aborts the feedback control over the secondary hydraulic pressure and, in replace of the feedback control, controls the secondary hydraulic pressure by an open control without using the actual secondary hydraulic pressure to acquire the command secondary hydraulic pressure, wherein during the belt slip control the command secondary pressure is oscillated for estimating the belt slip condition by the belt slip control means, and wherein during the belt slip control the actual secondary hydraulic pressure is decreased on the basis of the estimation of the belt slip control means so that the predetermined belt slip condition is maintained. a secondary hydraulic pressure control means configured to control a secondary hydraulic pressure to the secondary pulley on the basis of a command secondary hydraulic pressure to the secondary pulley, and characterized by a belt slip control means configured to estimate a belt slip condition by monitoring a phase difference between an oscillation component included in the actual secondary hydraulic pressure and an oscillation component included in the actual gear ratio, wherein the control device is configured to switch between a normal control and a belt slip control over the second hydraulic pressure, based on a belt slip control permission condition, wherein during the normal control the secondary hydraulic pressure control means determines a command secondary hydraulic pressure by a feedback control based on a deviation between a target secondary hydraulic pressure and an actual secondary hydraulic pressure, and wherein during the belt slip control the secondary hydraulic pressure control means aborts the feedback control over the secondary hydraulic pressure and, in replace of the feedback control, controls the secondary hydraulic pressure by an open control without using the actual secondary hydraulic pressure to acquire the command secondary hydraulic pressure, wherein during the belt slip control the command secondary pressure is oscillated for estimating the belt slip condition by the belt slip control means, and wherein during the belt slip control the actual secondary hydraulic pressure is decreased on the basis of the estimation of the belt slip control means so that the predetermined belt slip condition is maintained.