A hydraulic system of an industrial vehicle comprising: a loading actuator (9, 10) and a brake unit that are driven by hydraulic oil, the hydraulic system of an industrial vehicle includes: an oil tank in which the hydraulic oil is retained; a hydraulic pump (6, 16) configured to draw in the hydraulic oil from the oil tank and supply the hydraulic oil to the loading actuator (9, 10) and the brake unit; a control valve configured to control the hydraulic oil supplied from the hydraulic pump to the loading actuator (9, 10); characterised by a brake valve configured to control the hydraulic oil to be supplied to the brake unit according to an operation of a brake operation tool; a hydraulic oil cooler configured to cool the hydraulic oil; a cooling passage that allows the hydraulic oil to flow toward the hydraulic oil cooler and further allows the hydraulic oil cooled in the hydraulic oil cooler to flow toward the brake unit; and a switching valve (21, 50) configured to switch between a first oil passage (21a, 50a) that cuts off a supply of the hydraulic oil from the hydraulic pump to the cooling passage and a second oil passage (21b, 50b) that allows the supply of the hydraulic oil from the hydraulic pump to the cooling passage. a loading actuator (9, 10) and a brake unit that are driven by hydraulic oil, the hydraulic system of an industrial vehicle includes: an oil tank in which the hydraulic oil is retained; a hydraulic pump (6, 16) configured to draw in the hydraulic oil from the oil tank and supply the hydraulic oil to the loading actuator (9, 10) and the brake unit; a control valve configured to control the hydraulic oil supplied from the hydraulic pump to the loading actuator (9, 10); characterised by a brake valve configured to control the hydraulic oil to be supplied to the brake unit according to an operation of a brake operation tool; a hydraulic oil cooler configured to cool the hydraulic oil; a cooling passage that allows the hydraulic oil to flow toward the hydraulic oil cooler and further allows the hydraulic oil cooled in the hydraulic oil cooler to flow toward the brake unit; and a switching valve (21, 50) configured to switch between a first oil passage (21a, 50a) that cuts off a supply of the hydraulic oil from the hydraulic pump to the cooling passage and a second oil passage (21b, 50b) that allows the supply of the hydraulic oil from the hydraulic pump to the cooling passage. an oil tank in which the hydraulic oil is retained; a hydraulic pump (6, 16) configured to draw in the hydraulic oil from the oil tank and supply the hydraulic oil to the loading actuator (9, 10) and the brake unit; a control valve configured to control the hydraulic oil supplied from the hydraulic pump to the loading actuator (9, 10); characterised by a brake valve configured to control the hydraulic oil to be supplied to the brake unit according to an operation of a brake operation tool; a hydraulic oil cooler configured to cool the hydraulic oil; a cooling passage that allows the hydraulic oil to flow toward the hydraulic oil cooler and further allows the hydraulic oil cooled in the hydraulic oil cooler to flow toward the brake unit; and a switching valve (21, 50) configured to switch between a first oil passage (21a, 50a) that cuts off a supply of the hydraulic oil from the hydraulic pump to the cooling passage and a second oil passage (21b, 50b) that allows the supply of the hydraulic oil from the hydraulic pump to the cooling passage.