Simulation device, numerical control device, and simulation method

To provide easy determination of a causative factor of a machining problem in a machining target or easy checking of the effectiveness of an adjustment of, e.g., a machining program, a control command such as a position command, a servo control, or machine operation. This invention includes: storage units storing pieces of machining position data obtained from machining performed based on at least two of a machining program, a control command for performing servo control on a servo motor for driving a machine tool, and pieces of feedback information related to the servo control and machine operation, the machining being performed on a machining target by the machine tool; a machined surface simulation unit performing machined surface simulations based on the stored machining position data; and a display unit displaying images of machined surfaces resulting from the machined surface simulations in a juxtaposed manner.