To provide work machine that even in a state in which ratio between rotating speed of an engine and rotating speed of an electric motor varies, can activate a hydraulic pump while keeping energy loss to a minimum.

A hydraulic shovel that is the work machine, comprises: a work device 3 attached to a rotary body 2; an engine 10 mounted on the rotary body 2; a hydraulic pump 11; an electric motor 12 capable of activating the hydraulic pump 11 and being activated as a power generator; and a battery 13 that supplies electricity for activating the electric motor 12 and stores electricity generated when activating the electric motor 12 as a power generator. Further, the hydraulic shovel comprises: an output shaft 14 of the engine 10; a differential gear mechanism 16 provided between an outer shaft 14 of the engine 10 and a rotation shaft 15 of the electric motor 12; a gear 17 provided integrally on the differential gear mechanism 16; a gear 19, engaging with the gear 17, that is provided integrally with an input shaft 18 of the hydraulic pump 11.