A numerical control device includes: a data storage unit that cyclically associates and stores command speeds and actual speeds of a motor in chronological order; a difference pattern table that associates and stores causes of a difference between the command speed and the actual speed, and difference patterns of the causes; a cause analyzing unit that analyzes the cause of the difference between the command speed and the actual speed by comparing the command speeds and the actual speeds stored in chronological order in the data storage unit, with the difference patterns stored in the difference pattern table; and a display unit that displays the analyzed cause.

A numerical control device includes: a data storage unit that cyclically associates and stores command speeds and actual speeds of a motor in chronological order; a difference pattern table that associates and stores causes of a difference between the command speed and the actual speed, and difference patterns of the causes; a cause analyzing unit that analyzes the cause of the difference between the command speed and the actual speed by comparing the command speeds and the actual speeds stored in chronological order in the data storage unit, with the difference patterns stored in the difference pattern table; and a display unit that displays the analyzed cause.