A laser device comprising: a laser oscillator configured to generate a laser beam ; a condenser lens configured to condense the laser beam emitted from the laser oscillator ; a transmission fiber including at least (i) a core configured to transmit a part of the laser beam condensed by the condenser lens and (ii) a cladding provided around the core ; a reflection light monitor that detects a second light intensity of a second part of the laser beam when the laser beam is incident on a laser beam incident part of the transmission fiber , the second light intensity including a third light intensity of a third part of the laser beam reflected or scattered at the laser beam incident part ; an output light monitor that detects a fifth light intensity of a fifth part of the laser beam emitted from the laser oscillator ; and a lens driving unit configured to automatically adjust a position of the condenser lens to reduce a first light intensity of a first part of the laser beam incident on the cladding , wherein the second light intensity detected by the reflection light monitor includes a fourth light intensity of a fourth part of the laser beam detected proportional to the output of the laser beam emitted from the laser oscillator , and the lens driving unit is configured to calculate the fourth light intensity detected proportional to the output of the laser beam emitted from the laser oscillator based on the fifth light intensity detected by the output light monitor , and to adjust the position of the condenser lens in an optical axis direction so as to reduce a sixth light intensity obtained by subtracting the calculated fourth light intensity from the second light intensity detected by the reflection light monitor . a laser oscillator configured to generate a laser beam ; a condenser lens configured to condense the laser beam emitted from the laser oscillator ; a transmission fiber including at least (i) a core configured to transmit a part of the laser beam condensed by the condenser lens and (ii) a cladding provided around the core ; a reflection light monitor that detects a second light intensity of a second part of the laser beam when the laser beam is incident on a laser beam incident part of the transmission fiber , the second light intensity including a third light intensity of a third part of the laser beam reflected or scattered at the laser beam incident part ; an output light monitor that detects a fifth light intensity of a fifth part of the laser beam emitted from the laser oscillator ; and a lens driving unit configured to automatically adjust a position of the condenser lens to reduce a first light intensity of a first part of the laser beam incident on the cladding , wherein the second light intensity detected by the reflection light monitor includes a fourth light intensity of a fourth part of the laser beam detected proportional to the output of the laser beam emitted from the laser oscillator , and the lens driving unit is configured to calculate the fourth light intensity detected proportional to the output of the laser beam emitted from the laser oscillator based on the fifth light intensity detected by the output light monitor , and to adjust the position of the condenser lens in an optical axis direction so as to reduce a sixth light intensity obtained by subtracting the calculated fourth light intensity from the second light intensity detected by the reflection light monitor . <EOS>