A prime mover control device for a work vehicle, includes: a rotation speed control unit that controls a rotation speed of a prime mover in correspondence to an operation quantity of an accelerator operation member; a temperature detection unit that detects a temperature of cooling oil used to cool a brake; and a speed limiting unit that limits a maximum rotation speed of the prime mover by setting a lower limit for the maximum rotation speed when the temperature of the cooling oil detected by the temperature detection unit is higher than a predetermined temperature, compared to a limit set when the temperature of the cooling oil detected by the temperature detection unit is lower than the predetermined temperature, wherein: a maximum vehicle speed is limited by limiting the maximum rotation speed of the prime mover by the speed limiting unit.