A hydraulic system for a work machine comprising: a first control valve (V1, V2) to control a first hydraulic actuator (C1, M1); a second control valve (V3-V11) to control a second hydraulic actuator (C1-C5, M2, M3); a tank to store an operation fluid; a first operation fluid tube to supply the operation fluid to the first control valve; a second operation fluid tube to supply the operation fluid to the second control valve; a first transmission fluid tube to transmit a load pressure of the first hydraulic actuator; a second transmission fluid tube to transmit a load pressure of the second hydraulic actuator; a first switch valve having: a confluent position to connect the first operation fluid tube to the second operation fluid tube and to connect the first transmission fluid tube to the second transmission fluid tube; and an isolation position to release the connection between the first operation fluid tube and the second operation fluid tube and to release the connection between the first transmission fluid tube and the second transmission fluid tube, the first switch valve being configured to be switched to the confluent position and to the isolation position; a first return circuit configured to be connected to the first transmission fluid tube and to return the operation fluid in the first transmission fluid tube to the tank at the isolation position; and a second return circuit to return the operation fluid in the second transmission fluid tube to the tank at the confluent position and the isolation position, characterized in that the first return circuit is configured to release the connection to the first transmission fluid tube at the confluent position. a first control valve (V1, V2) to control a first hydraulic actuator (C1, M1); a second control valve (V3-V11) to control a second hydraulic actuator (C1-C5, M2, M3); a tank to store an operation fluid; a first operation fluid tube to supply the operation fluid to the first control valve; a second operation fluid tube to supply the operation fluid to the second control valve; a first transmission fluid tube to transmit a load pressure of the first hydraulic actuator; a second transmission fluid tube to transmit a load pressure of the second hydraulic actuator; a first switch valve having: a confluent position to connect the first operation fluid tube to the second operation fluid tube and to connect the first transmission fluid tube to the second transmission fluid tube; and an isolation position to release the connection between the first operation fluid tube and the second operation fluid tube and to release the connection between the first transmission fluid tube and the second transmission fluid tube, the first switch valve being configured to be switched to the confluent position and to the isolation position; a first return circuit configured to be connected to the first transmission fluid tube and to return the operation fluid in the first transmission fluid tube to the tank at the isolation position; and a second return circuit to return the operation fluid in the second transmission fluid tube to the tank at the confluent position and the isolation position, a confluent position to connect the first operation fluid tube to the second operation fluid tube and to connect the first transmission fluid tube to the second transmission fluid tube; and an isolation position to release the connection between the first operation fluid tube and the second operation fluid tube and to release the connection between the first transmission fluid tube and the second transmission fluid tube, the first switch valve being configured to be switched to the confluent position and to the isolation position; a first return circuit configured to be connected to the first transmission fluid tube and to return the operation fluid in the first transmission fluid tube to the tank at the isolation position; and a second return circuit to return the operation fluid in the second transmission fluid tube to the tank at the confluent position and the isolation position, characterized in that the first return circuit is configured to release the connection to the first transmission fluid tube at the confluent position.