Method for controlling power transmission device and power transmission device

A control method for controlling a power transmission device that is provided with a continuously variable transmission mechanism for transmitting power between a drive source and a drive wheel, a first oil passage that connects a primary pulley oil chamber and a secondary pulley oil chamber of the continuously variable transmission mechanism, an electric oil pump provided in the first oil passage, a second oil passage that branches from the first oil passage between the electric oil pump and the primary pulley oil chamber and that is connected to an oil reservoir, a switching valve provided at a branch point between the first oil passage and the second oil passage, and a third oil passage that branches from the first oil passage between the electric oil pump and the secondary pulley oil chamber and reaches the switching valve,

• the switching valve switching between two positions, a first position in which at least the first oil passage is connected, and a second position in which the second oil passage connects with the first oil passage on a side of the secondary pulley oil chamber and in which the third oil passage and connects with the first oil passage on a side of the primary pulley oil chamber,
• the control method comprising:
• making a magnitude of a hydraulic pressure difference between the third oil passage and the first oil passage one the side of the primary pulley oil chamber smaller than a prescribed threshold value when switching the switching valve between the first position and the second position.