An information processing device includes a processor. The processor acquires an operation amount for acceleration/deceleration of a first vehicle by a driver of the first vehicle. The processor acquires an inter-vehicle distance from the first vehicle to a second vehicle traveling ahead of the first vehicle. The processor determines whether the operation amount meets a first specific condition determined in advance. The processor determines whether the inter-vehicle distance meets a second specific condition determined in advance. The processor calculates a diagnosis result about the inter-vehicle distance based on the inter-vehicle distance acquired when the processor determines that both the first specific condition and the second specific condition are met. The processor outputs the diagnosis result.