A soft sensor device that acquires a state variable x by calculation using a plurality of processor cores, in which a value of the state variable x changes in relation to an input variable u that is observable, and a time derivative dx/dt of the state variable x is expressed by a function f(x, g(x), u) having the state variable x, an inner function g(x) having the state variable x as an independent variable, and the input variable u, as independent variables, the soft sensor device, comprising: a first arithmetic operation device (100; 200; 300; 400) that is an arithmetic operation device configured to perform an arithmetic operation by using one or a plurality of processor cores, and is programmed to calculate a function f(x, v, u) by using respective values of the state variable x, an intervening variable v that is defined by the inner function g(x), and the input variable u, and further obtain a value of the state variable x by time-integrating a value of the function f(x, v, u); and a second arithmetic operation device (110; 210; 310 1 to 310 n; 410 1 to 410 n) that is an arithmetic operation device configured to perform an arithmetic operation by using one or a plurality of processor cores that is or are different from the processor core or the processor cores used in the first arithmetic operation device (100; 200; 300; 400), and is programmed to calculate the intervening variable v, wherein the first arithmetic operation device (100; 200; 300; 400) is programmed to calculate the function f(x, v, u) by using a value of the state variable x that is calculated in the first arithmetic operation device (100; 200; 300; 400) in processing of a previous time, a value of the intervening variable v that is calculated in the second arithmetic device in the processing of the previous time, and a value of the input variable u that is inputted in processing of this time. a first arithmetic operation device (100; 200; 300; 400) that is an arithmetic operation device configured to perform an arithmetic operation by using one or a plurality of processor cores, and is programmed to calculate a function f(x, v, u) by using respective values of the state variable x, an intervening variable v that is defined by the inner function g(x), and the input variable u, and further obtain a value of the state variable x by time-integrating a value of the function f(x, v, u); and a second arithmetic operation device (110; 210; 310 1 to 310 n; 410 1 to 410 n) that is an arithmetic operation device configured to perform an arithmetic operation by using one or a plurality of processor cores that is or are different from the processor core or the processor cores used in the first arithmetic operation device (100; 200; 300; 400), and is programmed to calculate the intervening variable v, wherein the first arithmetic operation device (100; 200; 300; 400) is programmed to calculate the function f(x, v, u) by using a value of the state variable x that is calculated in the first arithmetic operation device (100; 200; 300; 400) in processing of a previous time, a value of the intervening variable v that is calculated in the second arithmetic device in the processing of the previous time, and a value of the input variable u that is inputted in processing of this time.