A catalyst for producing acrolein and/or acrylic acid from propylene by gas-phase catalytic oxidation with molecular oxygen-containing gas, comprising a catalytic active component, wherein: the catalyst is prepared by a method comprising the step of adding an aqueous solution containing cobalt and bismuth to an aqueous solution containing molybdenum over a period of 30 seconds to 10 minutes; the catalytic active component has a composition expressed by the following formula; and the catalytic active component contains a crystalline part showing a peak at 2θ = 26.5 ± 0.5°, measured by an X-ray diffraction analysis with Cu-Kα radiation, and has a crystallinity T of 4% or more and 18% or less in the 2θ range of 5° to 90°, measured by an X-ray diffraction analysis with Cu-Kα radiation; and a crystallinity ratio R expressed by the following formula is in the range of 0.03 or more and 0.20 or less R = M / T wherein M represents a crystallinity of the peak at 2θ = 26.5 ± 0.5° in the catalytic active component, measured by the X-ray diffraction analysis with Cu-Kα radiation, MO a Bi b Co c X1 d X2 e X3 f X4 g O z wherein Mo is molybdenum; Bi is bismuth; Co is cobalt; X1 represents at least one kind of element selected from the group consisting of iron and nickel; X2 represents at least one kind of element selected from the group consisting of alkali metals, alkali earth metals, boron and thallium; X3 represents at least one kind of element selected from the group consisting of tungsten, silicon, aluminum, zirconium and titanium; X4 represents at least one kind of element selected from the group consisting of phosphorus, tellurium, antimony, tin, cerium, lead, niobium, manganese, arsenic and zinc; O is oxygen; a, b, c, d, e, f, g and z mean atomic ratios of Mo, Bi, Co, X1, X2, X3, X4 and O, respectively, and meet inequalities: a = 12, 0.1 ≤ b ≤ 10, 0.1 ≤ c ≤ 20, 0.1 ≤ d ≤ 20, 0.001 ≤ e ≤ 10, 0 ≤ f ≤ 30 and 0 ≤ g ≤ 10, respectively; and z is a numeral value determined by oxidation states of respective elements. the catalyst is prepared by a method comprising the step of adding an aqueous solution containing cobalt and bismuth to an aqueous solution containing molybdenum over a period of 30 seconds to 10 minutes; the catalytic active component has a composition expressed by the following formula; and the catalytic active component contains a crystalline part showing a peak at 2θ = 26.5 ± 0.5°, measured by an X-ray diffraction analysis with Cu-Kα radiation, and has a crystallinity T of 4% or more and 18% or less in the 2θ range of 5° to 90°, measured by an X-ray diffraction analysis with Cu-Kα radiation; and a crystallinity ratio R expressed by the following formula is in the range of 0.03 or more and 0.20 or less R = M / T wherein M represents a crystallinity of the peak at 2θ = 26.5 ± 0.5° in the catalytic active component, measured by the X-ray diffraction analysis with Cu-Kα radiation, MO a Bi b Co c X1 d X2 e X3 f X4 g O z wherein Mo is molybdenum; Bi is bismuth; Co is cobalt; X1 represents at least one kind of element selected from the group consisting of iron and nickel; X2 represents at least one kind of element selected from the group consisting of alkali metals, alkali earth metals, boron and thallium; X3 represents at least one kind of element selected from the group consisting of tungsten, silicon, aluminum, zirconium and titanium; X4 represents at least one kind of element selected from the group consisting of phosphorus, tellurium, antimony, tin, cerium, lead, niobium, manganese, arsenic and zinc; O is oxygen; a, b, c, d, e, f, g and z mean atomic ratios of Mo, Bi, Co, X1, X2, X3, X4 and O, respectively, and meet inequalities: a = 12, 0.1 ≤ b ≤ 10, 0.1 ≤ c ≤ 20, 0.1 ≤ d ≤ 20, 0.001 ≤ e ≤ 10, 0 ≤ f ≤ 30 and 0 ≤ g ≤ 10, respectively; and z is a numeral value determined by oxidation states of respective elements.