A construction machine which comprises: a lower propelling body; an upper slewing body including an upper frame and slewably mounted on the lower propelling body; a cab including a floor plate and being installed on the upper frame; and a cab noise reduction device, the cab noise reduction device comprising: the floor plate; an under-cab member including a cab deck supporting the cab and being provided under the cab to define a closed-form under-cab space whose periphery is substantially closed, in cooperation with the floor plate; characterized by a resonator provided in the under-cab space to reduce noise intruding into the under-cab space by a resonance effect, wherein the resonator includes: a hollow body portion having a sidewall; and an opening provided in the sidewall of the body portion to let an internal space of the body portion and the under-cab space outside the body portion communicate with each other through the opening; the undercab member has a hollow box-shaped portion whose periphery is substantially closed, and wherein an opening is provided in the hollow box-shaped portion to form the resonator including the hollow box-shaped portion as the body portion; the under-cab member includes a plurality of mounting seats each having a cab mounting member attached thereonto to support the cab from lower side thereof, and at least one of the mounting seats is formed into a hollow box shape whose periphery is substantially closed, to form the body portion. a lower propelling body; an upper slewing body including an upper frame and slewably mounted on the lower propelling body; a cab including a floor plate and being installed on the upper frame; and a cab noise reduction device, the cab noise reduction device comprising: the floor plate; an under-cab member including a cab deck supporting the cab and being provided under the cab to define a closed-form under-cab space whose periphery is substantially closed, in cooperation with the floor plate; characterized by a resonator provided in the under-cab space to reduce noise intruding into the under-cab space by a resonance effect, wherein the resonator includes: a hollow body portion having a sidewall; and an opening provided in the sidewall of the body portion to let an internal space of the body portion and the under-cab space outside the body portion communicate with each other through the opening; the undercab member has a hollow box-shaped portion whose periphery is substantially closed, and wherein an opening is provided in the hollow box-shaped portion to form the resonator including the hollow box-shaped portion as the body portion; the under-cab member includes a plurality of mounting seats each having a cab mounting member attached thereonto to support the cab from lower side thereof, and at least one of the mounting seats is formed into a hollow box shape whose periphery is substantially closed, to form the body portion. the floor plate; an under-cab member including a cab deck supporting the cab and being provided under the cab to define a closed-form under-cab space whose periphery is substantially closed, in cooperation with the floor plate; characterized by a resonator provided in the under-cab space to reduce noise intruding into the under-cab space by a resonance effect, wherein the resonator includes: a hollow body portion having a sidewall; and an opening provided in the sidewall of the body portion to let an internal space of the body portion and the under-cab space outside the body portion communicate with each other through the opening; the undercab member has a hollow box-shaped portion whose periphery is substantially closed, and wherein an opening is provided in the hollow box-shaped portion to form the resonator including the hollow box-shaped portion as the body portion; the under-cab member includes a plurality of mounting seats each having a cab mounting member attached thereonto to support the cab from lower side thereof, and at least one of the mounting seats is formed into a hollow box shape whose periphery is substantially closed, to form the body portion.