A variable stroke characteristic engine is provided in which a piston (11) and a crankshaft (30) are linked to a control shaft (65) via a variable stroke link mechanism (LV), and the variable stroke link mechanism (LV) is operated by an actuator (AC) that drives the control shaft (65) to thus make the stroke travel of the piston (11) variable, wherein a housing (HU) of the hydraulic actuator (AC) for operating the variable stroke link mechanism (LV) is provided in a bearing member (54) of a lower block (41) supporting the crankshaft (30), and the housing (HU) is thus made small, thereby suppressing any increase in the dimensions of the engine. Furthermore, a hydraulic switching valve unit (92) for controlling operation of the hydraulic actuator (AC) is directly mounted on the housing (HU), thereby enhancing the responsiveness of the hydraulic actuator (AC).