A machine tool having a tool changer and sequentially changing tools to machine a workpiece, the machine tool comprising: an input unit for inputting a machining program output from a CAM system and basic data of the tools; a storage unit storing the basic data of the tools input from the input unit for each tool no.; a tool measuring device measuring dimensions of the tools loaded in a tool magazine of the tool changer, and a judging unit comparing the basic data of the tools stored in the storage unit with actual measurement data measured by the measuring device and judging that a measured tool differs from a desired tool if a difference in dimensions between the measured and desired tools exceeds a predetermined allowable range; and a display unit displaying statuses of the tools, wherein the tool measuring device is configured to measure the tools in a first time period after they are loaded in the tool magazine and before they are used for machining, the judging unit is configured to judge that the measured tool is a mistakenly loaded improper tool and sends a first warning signal to the display unit of the machine tool if the difference in dimensions between the measured tool measured in the first time period and the desired tool exceeds the allowable range, the tool measuring device is configured to measure the tools in a second time period after they are used for machining and when they are returned to the tool magazine, the judging unit is configured to judge that the measured tool is a damaged tool and sends a second warning signal to the display unit if the difference in dimensions between the measured tool measured in the second time period and the desired tool exceeds the allowable range, and based on the first and second warning signals the display unit is configured to display the statuses of the tools which are different for the first and second warning signals. an input unit for inputting a machining program output from a CAM system and basic data of the tools; a storage unit storing the basic data of the tools input from the input unit for each tool no.; a tool measuring device measuring dimensions of the tools loaded in a tool magazine of the tool changer, and a judging unit comparing the basic data of the tools stored in the storage unit with actual measurement data measured by the measuring device and judging that a measured tool differs from a desired tool if a difference in dimensions between the measured and desired tools exceeds a predetermined allowable range; and a display unit displaying statuses of the tools, wherein the tool measuring device is configured to measure the tools in a first time period after they are loaded in the tool magazine and before they are used for machining, the judging unit is configured to judge that the measured tool is a mistakenly loaded improper tool and sends a first warning signal to the display unit of the machine tool if the difference in dimensions between the measured tool measured in the first time period and the desired tool exceeds the allowable range, the tool measuring device is configured to measure the tools in a second time period after they are used for machining and when they are returned to the tool magazine, the judging unit is configured to judge that the measured tool is a damaged tool and sends a second warning signal to the display unit if the difference in dimensions between the measured tool measured in the second time period and the desired tool exceeds the allowable range, and based on the first and second warning signals the display unit is configured to display the statuses of the tools which are different for the first and second warning signals.