PROBLEM TO BE SOLVED: To prevent a state in which a hydraulic cylinder is actuated to a descent side even though a control valve is operated to an ascent position by a lifting lever, when a hydraulic pump stops.SOLUTION: A work machine comprises an operation valve 36 having block positions 36a and 36b which allow supply of a hydraulic oil to a hydraulic cylinder 4 and block discharge of the hydraulic oil from the hydraulic cylinder 4, and an allowable position 36c which allows the discharge of the hydraulic oil from the hydraulic cylinder 4. When a control valve 20 is operated to an ascent position 20U and a neutral position 20N by a lifting lever 27, the operation valve 36 is operated to the block positions 36a and 36b. When the control valve 20 is operated to a descent position 20D by the lifting lever 27, the operation valve 36 is operated to the allowable position 36c.SELECTED DRAWING: Figure 6COPYRIGHT: (C)2016,JPO&INPIT【課題】油圧ポンプが停止した場合、昇降レバーにより制御弁を上昇位置に操作しているのに油圧シリンダが下降側に作動するような状態を防止する。【解決手段】油圧シリンダ4への作動油の供給を許容し且つ油圧シリンダ4からの作動油の排出を遮断する阻止位置36a,36b、及び、油圧シリンダ4からの作動油の排出を許容する許容位置36cを備えた操作弁36を備える。昇降レバー27により制御弁20を上昇位置20U及び中立位置20Nに操作すると、操作弁36が阻止位置36a,36bに操作され、昇降レバー27により制御弁20を下降位置20Dに操作すると、操作弁36が許容位置36cに操作されるように構成する。【選択図】図6
