A wheel loader comprising: an engine; a drive shaft (9, 12); a transmission comprising an input shaft through which a power from the engine is inputted and an output shaft through which the power from the engine is outputted to the drive shaft (9, 12), the transmission being configured to perform a gear-shifting operation between the input shaft and the output shaft; a mechanical drive unit comprising a plurality of gear trains driven by the power from the engine; a mechanical drive clutch (43, 47) provided to the mechanical drive unit to control transmission and cutoff of the power from the engine; a variable displacement hydraulic pump configured to be driven by the power from the engine to deliver a hydraulic oil for traveling; a variable displacement hydraulic motor (23, 24) configured to be driven by the hydraulic oil delivered from the hydraulic pump; a hydraulic drive unit comprising a plurality of gear trains driven by a power from the hydraulic motor (23, 24); a hydraulic drive clutch (54, 65) provided to the hydraulic drive unit to control transmission and cutoff of the power from the hydraulic motor (23, 24); a load detecting unit configured to detect an amount of a load driven by the power from the engine; a vehicle speed detecting unit configured to detect a vehicle speed; a switching controlling unit configured to set a switching vehicle speed in accordance with the amount of the load, the switching controlling unit being configured to set the switching vehicle speed at a high-speed side when the load becomes large and to set the switching vehicle speed at a low-speed side when the load becomes small; and a controller configured to switch between a hydraulic drive state where the output shaft is driven by the hydraulic drive unit and a mechanical drive state where the output shaft is driven by the mechanical drive unit based on the switching controlling unit, the controller being configured to control the mechanical drive clutch (43, 47) and the hydraulic drive clutch (54, 65) to switch from the hydraulic drive state to the mechanical drive state when the vehicle speed detected by the vehicle speed detecting unit reaches the switching vehicle speed set by the switching controlling unit in accordance with the load detected by the load detecting unit during acceleration of the wheel loader in the hydraulic drive state, wherein the controller is configured to lower an engine speed and increase a displacement of the hydraulic pump and/or reduce a displacement of the hydraulic motor (23, 24) before switching from the hydraulic drive state to the mechanical drive state. an engine; a drive shaft (9, 12); a transmission comprising an input shaft through which a power from the engine is inputted and an output shaft through which the power from the engine is outputted to the drive shaft (9, 12), the transmission being configured to perform a gear-shifting operation between the input shaft and the output shaft; a mechanical drive unit comprising a plurality of gear trains driven by the power from the engine; a mechanical drive clutch (43, 47) provided to the mechanical drive unit to control transmission and cutoff of the power from the engine; a variable displacement hydraulic pump configured to be driven by the power from the engine to deliver a hydraulic oil for traveling; a variable displacement hydraulic motor (23, 24) configured to be driven by the hydraulic oil delivered from the hydraulic pump; a hydraulic drive unit comprising a plurality of gear trains driven by a power from the hydraulic motor (23, 24); a hydraulic drive clutch (54, 65) provided to the hydraulic drive unit to control transmission and cutoff of the power from the hydraulic motor (23, 24); a load detecting unit configured to detect an amount of a load driven by the power from the engine; a vehicle speed detecting unit configured to detect a vehicle speed; a switching controlling unit configured to set a switching vehicle speed in accordance with the amount of the load, the switching controlling unit being configured to set the switching vehicle speed at a high-speed side when the load becomes large and to set the switching vehicle speed at a low-speed side when the load becomes small; and a controller configured to switch between a hydraulic drive state where the output shaft is driven by the hydraulic drive unit and a mechanical drive state where the output shaft is driven by the mechanical drive unit based on the switching controlling unit, the controller being configured to control the mechanical drive clutch (43, 47) and the hydraulic drive clutch (54, 65) to switch from the hydraulic drive state to the mechanical drive state when the vehicle speed detected by the vehicle speed detecting unit reaches the switching vehicle speed set by the switching controlling unit in accordance with the load detected by the load detecting unit during acceleration of the wheel loader in the hydraulic drive state, wherein the controller is configured to lower an engine speed and increase a displacement of the hydraulic pump and/or reduce a displacement of the hydraulic motor (23, 24) before switching from the hydraulic drive state to the mechanical drive state.