A prime mover control device for a work vehicle, comprising: a rotation speed control unit that controls a rotation speed of a prime mover in correspondence to an operation quantity at an accelerator operation member; a temperature detection unit (163a, 163b) that detects a temperature of cooling oil used to cool a brake; a speed limiting unit (100, 200, 300) that limits a target engine rotation speed (Nt) of the prime mover; and a vehicle speed detection unit that detects a vehicle speed; a maximum rotation speed of the prime mover being limited by limiting the target engine rotation speed (Nt), characterized in that the speed limiting unit (100, 200, 300) is adapted to: set the target engine rotation speed (Nt) of the prime mover to a limit value (Nxc) stored as a value lower than an upper limit value, which is the target engine rotation speed (Nt) at a time that the accelerator operation member is operated to a maximum extent, so as to limit the maximum rotation speed of the prime mover not to exceed the limit value (Nxc) when the temperature (Tb) of the cooling oil detected by the temperature detection unit (163a, 163b) is higher than a predetermined temperature (Tbl) and the vehicle speed detected by the vehicle speed detection unit is higher than a predetermined vehicle speed; and set the target engine rotation speed (Nt) of the prime mover to the upper limit value so as to set the maximum rotation speed of the prime mover when the temperature (Tb) of the cooling oil detected by the temperature detection unit (163a, 163b) is lower than the predetermined temperature (Tbl), and when the temperature (Tb) of the cooling oil detected by the temperature detection unit (163a, 163b) is higher than the predetermined temperature (Tbl) and the vehicle speed detected by the vehicle speed detection unit is lower than the predetermined vehicle speed. a rotation speed control unit that controls a rotation speed of a prime mover in correspondence to an operation quantity at an accelerator operation member; a temperature detection unit (163a, 163b) that detects a temperature of cooling oil used to cool a brake; a speed limiting unit (100, 200, 300) that limits a target engine rotation speed (Nt) of the prime mover; and a vehicle speed detection unit that detects a vehicle speed; a maximum rotation speed of the prime mover being limited by limiting the target engine rotation speed (Nt), characterized in that the speed limiting unit (100, 200, 300) is adapted to: set the target engine rotation speed (Nt) of the prime mover to a limit value (Nxc) stored as a value lower than an upper limit value, which is the target engine rotation speed (Nt) at a time that the accelerator operation member is operated to a maximum extent, so as to limit the maximum rotation speed of the prime mover not to exceed the limit value (Nxc) when the temperature (Tb) of the cooling oil detected by the temperature detection unit (163a, 163b) is higher than a predetermined temperature (Tbl) and the vehicle speed detected by the vehicle speed detection unit is higher than a predetermined vehicle speed; and set the target engine rotation speed (Nt) of the prime mover to the upper limit value so as to set the maximum rotation speed of the prime mover when the temperature (Tb) of the cooling oil detected by the temperature detection unit (163a, 163b) is lower than the predetermined temperature (Tbl), and when the temperature (Tb) of the cooling oil detected by the temperature detection unit (163a, 163b) is higher than the predetermined temperature (Tbl) and the vehicle speed detected by the vehicle speed detection unit is lower than the predetermined vehicle speed. set the target engine rotation speed (Nt) of the prime mover to a limit value (Nxc) stored as a value lower than an upper limit value, which is the target engine rotation speed (Nt) at a time that the accelerator operation member is operated to a maximum extent, so as to limit the maximum rotation speed of the prime mover not to exceed the limit value (Nxc) when the temperature (Tb) of the cooling oil detected by the temperature detection unit (163a, 163b) is higher than a predetermined temperature (Tbl) and the vehicle speed detected by the vehicle speed detection unit is higher than a predetermined vehicle speed; and set the target engine rotation speed (Nt) of the prime mover to the upper limit value so as to set the maximum rotation speed of the prime mover when the temperature (Tb) of the cooling oil detected by the temperature detection unit (163a, 163b) is lower than the predetermined temperature (Tbl), and when the temperature (Tb) of the cooling oil detected by the temperature detection unit (163a, 163b) is higher than the predetermined temperature (Tbl) and the vehicle speed detected by the vehicle speed detection unit is lower than the predetermined vehicle speed.