A monitoring apparatus for doctoring in a manufacturing process of an end product of a fiber web machine, comprising: at least one vibration sensor (100A, 100A', 100A", 100B, 100B') each of which is attached at a location of at least one doctor blade arrangement (10, 10'), the at least one vibration sensor (100A, 100A', 100A", 100B, 100B') being configured to convert a mechanical vibration of the at least one doctor blade arrangement (10, 10') to an electrical signal, the mechanical vibration being caused by a doctor blade, in response to a transfer of a front end of a moving web to a rotating roll and a scraping operation of a rotating roll which is in contact with the moving web, the scraping of said doctor blade removing material that is used in the manufacturing process of the end product from the roll; and a data processing unit configured to receive the electrical signal carrying information on the vibration; characterised in that: said doctor blade is a single doctor blade, or the first doctor blade if there are more than one doctor blades one after another; and in that the data processing unit is further configured to compare said vibration with a reference of at least one previous measurement of a same grade of the web, predict a drop-through is about to take place and start one or more preventive actions in order to avoid the drop-through and damages of the drop-through to the fiber web machine in the case said vibration exceeds at least one boundary of the reference. at least one vibration sensor (100A, 100A', 100A", 100B, 100B') each of which is attached at a location of at least one doctor blade arrangement (10, 10'), the at least one vibration sensor (100A, 100A', 100A", 100B, 100B') being configured to convert a mechanical vibration of the at least one doctor blade arrangement (10, 10') to an electrical signal, the mechanical vibration being caused by a doctor blade, in response to a transfer of a front end of a moving web to a rotating roll and a scraping operation of a rotating roll which is in contact with the moving web, the scraping of said doctor blade removing material that is used in the manufacturing process of the end product from the roll; and a data processing unit configured to receive the electrical signal carrying information on the vibration; characterised in that: said doctor blade is a single doctor blade, or the first doctor blade if there are more than one doctor blades one after another; and in that the data processing unit is further configured to compare said vibration with a reference of at least one previous measurement of a same grade of the web, predict a drop-through is about to take place and start one or more preventive actions in order to avoid the drop-through and damages of the drop-through to the fiber web machine in the case said vibration exceeds at least one boundary of the reference. said doctor blade is a single doctor blade, or the first doctor blade if there are more than one doctor blades one after another; and in that the data processing unit is further configured to compare said vibration with a reference of at least one previous measurement of a same grade of the web, predict a drop-through is about to take place and start one or more preventive actions in order to avoid the drop-through and damages of the drop-through to the fiber web machine in the case said vibration exceeds at least one boundary of the reference.