To reduce the size and weight of a wall decontaminating device and save its energy by reciprocating its brushes with air cylinders.

A wall decontaminating device A comprises angular brushes 3a and 3b and auxiliary suckers 4a and 4b placed at the respective upper and lower ends of its main body part 1 set in an angular frame 2 that serves also as a feed pipe, and a vertical main suction part 6 that includes main suckers 5. The decontaminating device A is adapted to move through alternate suction and shift motion of the main suckers 5 in the main suction part 6 and the auxiliary suckers 4a and 4b. The brushes 3a and 3b are attached to the ends of the piston rods of a pneumatic double-acting and double-rod cylinder. The position of the cylinder piston is detected with a sensor before it reaches its stroke ends, and pressure air is selectively allowed into or out of the cylinder by a solenoid valve controlled by such detection signals to thus reciprocate the cylinder at higher speed and simultaneously relieve its shocks at the stroke ends.