Vehicle travel control method and vehicle travel control device

A vehicle travel control device comprising:

• a vehicle speed sensor configured to detect a vehicle speed of a vehicle equipped with the vehicle travel control device;
• and
• at least one electronic controller including
• a constant speed travel control unit configured to execute a constant speed travel control;
• a downshift control unit configured to execute a downshift control of an automatic transmission upon determining a vehicle speed detected by the vehicle speed sensor increases from a set vehicle speed during the constant speed travel control of the constant speed travel control unit by at least a first prescribed value,
• a downshift prohibition control unit configured to prohibit a downshift control of the automatic transmission upon determining the vehicle speed has increased from the set vehicle speed by at least the first prescribed value due to an operation of an operating element by a driver during the constant speed travel control, and
• a re-downshift control unit configured to execute the downshift control of the automatic transmission upon determining an insufficient deceleration of the vehicle exists where the driver is not operating an accelerator pedal during the prohibiting of the downshift control by the downshift prohibition control unit,
• the insufficient deceleration of the vehicle being determined to exist when the vehicle speed increases from a reference vehicle speed by at least a second prescribed value where the reference vehicle speed is set to a lowest vehicle speed following a completion of the driver's operation.