1. A vehicle acceleration suppression device, comprising: an in-vehicle sonar configured to emit a transmission wave forward in a travel direction of a vehicle; an obstacle distance detector configured to receive a reflection wave of the transmission wave emitted by the in-vehicle sonar, and to detect a distance between the vehicle and an obstacle existing ahead in the travel direction of the vehicle; an obstacle height determination unit configured to receive the reflection wave of the transmission wave emitted by the in-vehicle sonar, and to determine whether or not the obstacle existing ahead in the travel direction of the vehicle is a tall obstacle that is as tall as the vehicle or taller than the vehicle; an obstacle type determination unit configured to determine a type of the obstacle based on at least a first-time detection distance that is the distance between the vehicle and the obstacle detected by the obstacle distance detector for a first time while the vehicle is approaching the obstacle, and a height determination distance that is the distance between the vehicle and the obstacle detected by the obstacle distance detector when the obstacle distance detector determines that the obstacle is the tall obstacle for the first time while the vehicle is approaching the obstacle; and an acceleration suppression controller configured to carry out acceleration suppression control for suppressing acceleration of the vehicle in a control manner depending on a determination result made by the obstacle type determination unit, when it is determined that the vehicle has approached the obstacle at an approach degree equal to or higher than a predefined approach degree based on the distance between the vehicle and the obstacle detected by the obstacle distance detector.