An optical transmitter module comprising: a housing that houses a semiconductor laser element for modulating a laser beam by an electrical signal; lead pins for electrically connecting the inside and outside of the housing; and a flexible printed circuit board connected to the lead pins, wherein said flexible printed circuit board includes: an insulating substrate; a signal pattern to be connected to the semiconductor laser element, which is formed on a first surface of the insulating substrate; first and second ground conductor patterns (105-1, 105-2) provided on both sides of the signal pattern; a third ground conductor pattern which is formed on a second surface of the insulating substrate; a first insulating layer for covering the signal pattern as well as the first and second ground conductor patterns; a second insulating layer for covering the third ground conductor pattern; a first covering conductive layer for covering the first insulating layer; and a second covering conductive layer for covering the second insulating layer, and wherein said first covering conductive layer is formed over the first and second ground conductor patterns and has a slit formed over and along the extensional direction of the signal pattern from one end of the first covering conductive layer to the other end of the first covering conductive layer. a housing that houses a semiconductor laser element for modulating a laser beam by an electrical signal; lead pins for electrically connecting the inside and outside of the housing; and a flexible printed circuit board connected to the lead pins, wherein said flexible printed circuit board includes: an insulating substrate; a signal pattern to be connected to the semiconductor laser element, which is formed on a first surface of the insulating substrate; first and second ground conductor patterns (105-1, 105-2) provided on both sides of the signal pattern; a third ground conductor pattern which is formed on a second surface of the insulating substrate; a first insulating layer for covering the signal pattern as well as the first and second ground conductor patterns; a second insulating layer for covering the third ground conductor pattern; a first covering conductive layer for covering the first insulating layer; and a second covering conductive layer for covering the second insulating layer, and wherein said first covering conductive layer is formed over the first and second ground conductor patterns and has a slit formed over and along the extensional direction of the signal pattern from one end of the first covering conductive layer to the other end of the first covering conductive layer. an insulating substrate; a signal pattern to be connected to the semiconductor laser element, which is formed on a first surface of the insulating substrate; first and second ground conductor patterns (105-1, 105-2) provided on both sides of the signal pattern; a third ground conductor pattern which is formed on a second surface of the insulating substrate; a first insulating layer for covering the signal pattern as well as the first and second ground conductor patterns; a second insulating layer for covering the third ground conductor pattern; a first covering conductive layer for covering the first insulating layer; and a second covering conductive layer for covering the second insulating layer, and wherein said first covering conductive layer is formed over the first and second ground conductor patterns and has a slit formed over and along the extensional direction of the signal pattern from one end of the first covering conductive layer to the other end of the first covering conductive layer.