A 3 sensor MOS (medical observation system) used during a medical procedure, comprising: a first prism that has a first reflection film (DYM1) which is configured to reflect IR light (N1) of light from an observation part and that is configured to cause a first image sensor to receive the IR light (N1); a second prism that has a second reflection film (BSP1) which is configured to reflect A% (A: a predetermined real number) visible light (V3) transmitted through the first reflection film (DYM1) and which is configured to transmit remaining (100 - A)% visible light (V4) and that is configured to cause a second image sensor to receive the A% visible light (V3); a third prism that is configured to cause a third image sensor to receive the (100 - A)% visible light (V4) transmitted through the second reflection film (BSP1); and a video signal processor that is configured to generate a first video signal (N1VD), a second video signal (V3VD), and a third video signal (V4VD) of the observation part based on respective imaging outputs (N1V,V3V,V4V) of the first image sensor , the second image sensor , and the third image sensor , and to combine the first video signal (N1VD), the second video signal (V3VD), and the third video signal (V4VD), wherein the video signal processor is configured to perform pixel shifting on one of the second video signal (V3VD) and the third video signal (V4VD) both having substantially same brightness as each other to generate a fourth video signal (VVDA) and to output a video signal (IMVVDA) obtained by combining the fourth video signal (VVDA) and the first video signal (N1VD) to a monitor (MN1). a first prism that has a first reflection film (DYM1) which is configured to reflect IR light (N1) of light from an observation part and that is configured to cause a first image sensor to receive the IR light (N1); a second prism that has a second reflection film (BSP1) which is configured to reflect A% (A: a predetermined real number) visible light (V3) transmitted through the first reflection film (DYM1) and which is configured to transmit remaining (100 - A)% visible light (V4) and that is configured to cause a second image sensor to receive the A% visible light (V3); a third prism that is configured to cause a third image sensor to receive the (100 - A)% visible light (V4) transmitted through the second reflection film (BSP1); and a video signal processor that is configured to generate a first video signal (N1VD), a second video signal (V3VD), and a third video signal (V4VD) of the observation part based on respective imaging outputs (N1V,V3V,V4V) of the first image sensor , the second image sensor , and the third image sensor , and to combine the first video signal (N1VD), the second video signal (V3VD), and the third video signal (V4VD), wherein the video signal processor is configured to perform pixel shifting on one of the second video signal (V3VD) and the third video signal (V4VD) both having substantially same brightness as each other to generate a fourth video signal (VVDA) and to output a video signal (IMVVDA) obtained by combining the fourth video signal (VVDA) and the first video signal (N1VD) to a monitor (MN1). <EOS>