A raindrop detecting device includes a raindrop sensor, and a controller. The raindrop sensor is disposed in a wiping area of a wiper blade on a front windshield, and outputs a detection signal regarding a raindrop amount. The controller activates the wiper blade based on the detection signal, and sets a prohibition period for which the raindrop sensor is prohibited from determining the raindrop amount. The raindrop sensor outputs signals when raindrop collected by the wiper blade passes above the raindrop sensor in a reciprocating wiping operation. The controller calculates the prohibition period of a second reciprocating wiping operation based on the signals of a first reciprocating wiping operation.