Safety device of vehicle with aerial work platform

A safety device of a vehicle with aerial work platform which comprises a traveling body that can travel, an elevating machine provided on the traveling body, and a work platform supported by the elevating machine, and which is configured to be able to move up and down the work platform by extending and contracting a rise and fall hydraulic cylinder which is a part of the elevating machine, comprising:

• a hydraulic pump that discharges hydraulic fluid to be supplied to the rise and fall hydraulic cylinder;
• a first switching valve that switches a bottom-side oil chamber of the rise and fall hydraulic cylinder between being connected to the hydraulic pump and being connected to an hydraulic fluid reservoir;
• a second switching valve provided in a line leading from the first switching valve to the bottom-side oil chamber of the rise and fall hydraulic cylinder and that switches between a first state of allowing an hydraulic fluid flow from the first switching valve side to the bottom-side oil chamber side while blocking a flow in an opposite direction and a second state of allowing both the flows;
• a check valve provided in a line leading from the hydraulic pump to the first switching valve and that allows an hydraulic fluid flow from the hydraulic pump side to the first switching valve side while blocking a flow in an opposite direction;
• a rise and fall control device that controls the first switching valve and the second switching valve to switch according to an operation instruction from a rise and fall operating device so as to make the rise and fall hydraulic cylinder extend and contract;
• and
• a rise and fall position detecting device that detects a rise and fall position of the work platform,
• wherein while not receiving an operation instruction to move down the work platform from the rise and fall operating device, when detecting the work platform moving down based on the detecting result of the rise and fall position detecting device, the rise and fall control device controls the first switching valve to switch so that the bottom-side oil chamber is connected to the hydraulic pump and controls the second switching valve to switch to the first state so as to make oil pressure in the bottom-side oil chamber be kept by the second switching valve and the check valve, thereby stopping the rise and fall hydraulic cylinder contracting.