A control system for an electronically controlled continuously variable transmission of a straddle type vehicle in which an intentional shift down operation is easily made in an AT mode. An MT mode temporarily selecting section makes the gear change mode selecting section select the MT mode temporarily and makes the gear ratio control section change the gear ratio of the transmission to a gear ratio from among a plurality of predetermined gear ratios that is positioned lower than a current gear ratio when a mode change signal is inputted in AT mode. An AT mode returning section returns the gear change mode selected by the gear change mode selecting section to AT mode when a release signal is outputted after MT mode is temporarily selected by the MT mode temporarily selecting section.