To provide a simple constitution capable of locking a step at a vehicle side advancing position and a retreating position, in a step device for a platform for preventing a passenger from falling between the platform and a stopped vehicle.

This device is equipped with a step 7, and a drive means (motor unit) 17 advancing and retreating the step 7 towards a vehicle side. The device also comprises a lock device 33 capable of locking the step 7 at the advancing position in cooperation with the motor unit 17, and locking it at the retreating position. The lock device 33 comprises an arm 34 swingably supported, and a solenoid 35 to be a swing means for swinging the arm 34. The arm 34 can be engaged with both of the step 7 at the retreating position, and the step 7 at the advancing position.