A device and method for detecting nozzle malfunction in an ink jet printhead having a predetermined number of nozzles. The printhead is first controlled whereby each of the nozzles is activated once in a manner which is required to print on a print medium a line having a number of ink dots equal to the predetermined number. Then a photodetector detects whether each location on the print medium which opposed one of the nozzles at the time of activation of that one nozzle has an ink dot printed thereat. Each location is photodetected in time sequence by scanning. The photodetector outputs a signal having a first level in response to each photodetected location which has no ink dot thereat and a signal having a second level in response to each photodetected location which has an ink dot thereat. The output of the photodetector is then processed. In accordance with one embodiment, the length of a printed line is effectively measured to determine if it is shorter than an expected length corresponding to a printed line formed by dots equal in number to the number of nozzles. In accordance with another embodiment, a multi-bit word is formed, each bit representing the state of a corresponding nozzle. The word is input to a look-up table to generate a status signal, in response to which further action will be taken.