A diesel engine is provided with an exhaust after-treatment device for purifying exhaust gas. The exhaust after-treatment device comprises a DPF that collects a particulate matter included in the exhaust gas, and an SCR that reduces nitrogen oxides included in the exhaust gas through addition of urea. The DPF is arranged to extend in an engine front-rear direction, which is a direction parallel to a crankshaft (CS), while the SCR is arranged to extend in an engine width direction rearward of the DPF in the engine front-rear direction.