A catheter assembly (10, 110, 210, 310, 510, 610, 710, 810, 910, 1010) comprising: an inner needle; a catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030) having a main lumen (34, 134, 234, 334, α, 534, 634, 734, 834, 934, 1034) through which the inner needle is detachably inserted and one or more sub lumens (41, 141, 241, 341, β, β1, β2, 540, 545, 641, 741, 841, 941) provided separately from the main lumen (34, 134, 234, 334, α, 534, 634, 734, 834, 934, 1034); characterized in that a deflection suppression mechanism (90, 590, 690, 790, 890, 990, 1090) is provided which is configured to suppress deflection of the inner needle by supporting the inner needle via the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030), wherein the one or more sub lumens (41, 141, 241, 341, β, β1, β2, 540, 545, 641, 741, 841, 941) respectively communicate with one or more sub openings (41a, 141a, 241a, 341a, 540a, 540b, 545a, 545b, 641a, 741a, 841a, 941a, 1040a, 1040b, 1045a, 1045b) formed in the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030), the deflection suppression mechanism (90, 590, 690, 790, 890, 990, 1090) has a sliding contact support portion (92, 595, 691, 791, 891, 991, 1092) rubbing against the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030) when the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030) advances with respect to the inner needle, and the sliding contact support portion (92, 595, 691, 791, 891, 991, 1092) is capable of supporting a proximal end side beyond the sub opening (41a, 141a, 241a, 341a, 540a, 540b, 545a, 545b, 641a, 741a, 841a, 941a, 1040a, 1040b, 1045a, 1045b) on a most distal side among the one or more sub openings (41a, 141a, 241a, 341a, 540a, 540b, 545a, 545b, 641a, 741a, 841a, 941a, 1040a, 1040b, 1045a, 1045b) or a proximal end side beyond a step portion (45, 145, 245, 345, 541, 546) generated in the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030) as the sub lumen (41, 141, 241, 341, β, β1, β2, 540, 545, 641, 741, 841, 941) is formed. an inner needle; a catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030) having a main lumen (34, 134, 234, 334, α, 534, 634, 734, 834, 934, 1034) through which the inner needle is detachably inserted and one or more sub lumens (41, 141, 241, 341, β, β1, β2, 540, 545, 641, 741, 841, 941) provided separately from the main lumen (34, 134, 234, 334, α, 534, 634, 734, 834, 934, 1034); characterized in that a deflection suppression mechanism (90, 590, 690, 790, 890, 990, 1090) is provided which is configured to suppress deflection of the inner needle by supporting the inner needle via the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030), wherein the one or more sub lumens (41, 141, 241, 341, β, β1, β2, 540, 545, 641, 741, 841, 941) respectively communicate with one or more sub openings (41a, 141a, 241a, 341a, 540a, 540b, 545a, 545b, 641a, 741a, 841a, 941a, 1040a, 1040b, 1045a, 1045b) formed in the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030), the deflection suppression mechanism (90, 590, 690, 790, 890, 990, 1090) has a sliding contact support portion (92, 595, 691, 791, 891, 991, 1092) rubbing against the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030) when the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030) advances with respect to the inner needle, and the sliding contact support portion (92, 595, 691, 791, 891, 991, 1092) is capable of supporting a proximal end side beyond the sub opening (41a, 141a, 241a, 341a, 540a, 540b, 545a, 545b, 641a, 741a, 841a, 941a, 1040a, 1040b, 1045a, 1045b) on a most distal side among the one or more sub openings (41a, 141a, 241a, 341a, 540a, 540b, 545a, 545b, 641a, 741a, 841a, 941a, 1040a, 1040b, 1045a, 1045b) or a proximal end side beyond a step portion (45, 145, 245, 345, 541, 546) generated in the catheter (30, 130, 230, 330, 430A to 430H, 530, 630, 730, 830, 930, 1030) as the sub lumen (41, 141, 241, 341, β, β1, β2, 540, 545, 641, 741, 841, 941) is formed.