An actuator comprising: a housing having an operating chamber; a fixed element mounted in the operating chamber; and a movable element to be driven by a driving source, wherein the movable element and the fixed element are formed by an iron-base sintered part comprising: a carburization quenched structure formed by an iron-nickel-molybdenum-carbon-based sintered alloy, the carburization quenched structure having density of 7.25 g/cm 3 or more, furthermore wherein when the iron-base sintered part forming the movable element is set to 100 % with regard to a mass%, the iron-base sintered part forming the movable element consists essentially of nickel of 0.5 to 5.5 %, molybdenum 0.1 to 1.0 %, copper 0.5 to 2.0 %, carbon 0.1 to 0.8 %, and a remainder containing substantially iron and inevitable impurities, and wherein when the iron-base sintered part forming the fixed element is set to 100 % with regard to a mass%, the iron-base sintered part forming the fixed element comprises nickel of 0.5 to 5.0 %, molybdenum 0.5 to 1.5 %, copper of 0 to 2.0 %, carbon 0.1 to 0.8 %, and a remainder containing substantially iron and inevitable impurities wherein the movable element is moved in contact with at least a portion of the fixed element and a mass ratio (Ni content in the movable element / Ni content in the fixed element) is in the range of 0.8 to 3. a housing having an operating chamber; a fixed element mounted in the operating chamber; and a movable element to be driven by a driving source, wherein the movable element and the fixed element are formed by an iron-base sintered part comprising: a carburization quenched structure formed by an iron-nickel-molybdenum-carbon-based sintered alloy, the carburization quenched structure having density of 7.25 g/cm 3 or more, furthermore wherein when the iron-base sintered part forming the movable element is set to 100 % with regard to a mass%, the iron-base sintered part forming the movable element consists essentially of nickel of 0.5 to 5.5 %, molybdenum 0.1 to 1.0 %, copper 0.5 to 2.0 %, carbon 0.1 to 0.8 %, and a remainder containing substantially iron and inevitable impurities, and wherein when the iron-base sintered part forming the fixed element is set to 100 % with regard to a mass%, the iron-base sintered part forming the fixed element comprises nickel of 0.5 to 5.0 %, molybdenum 0.5 to 1.5 %, copper of 0 to 2.0 %, carbon 0.1 to 0.8 %, and a remainder containing substantially iron and inevitable impurities wherein the movable element is moved in contact with at least a portion of the fixed element and a mass ratio (Ni content in the movable element / Ni content in the fixed element) is in the range of 0.8 to 3. a carburization quenched structure formed by an iron-nickel-molybdenum-carbon-based sintered alloy, the carburization quenched structure having density of 7.25 g/cm 3 or more, furthermore wherein when the iron-base sintered part forming the movable element is set to 100 % with regard to a mass%, the iron-base sintered part forming the movable element consists essentially of nickel of 0.5 to 5.5 %, molybdenum 0.1 to 1.0 %, copper 0.5 to 2.0 %, carbon 0.1 to 0.8 %, and a remainder containing substantially iron and inevitable impurities, and wherein when the iron-base sintered part forming the fixed element is set to 100 % with regard to a mass%, the iron-base sintered part forming the fixed element comprises nickel of 0.5 to 5.0 %, molybdenum 0.5 to 1.5 %, copper of 0 to 2.0 %, carbon 0.1 to 0.8 %, and a remainder containing substantially iron and inevitable impurities wherein the movable element is moved in contact with at least a portion of the fixed element and a mass ratio (Ni content in the movable element / Ni content in the fixed element) is in the range of 0.8 to 3.