A control apparatus for an internal combustion engine, comprising: an in-cylinder pressure sensor that is provided in at least one cylinder of an internal combustion engine, and that detects an in-cylinder pressure of the cylinder; rotation angle detection means that detects an angle to which a crankshaft of the internal combustion engine is rotated; pressure ratio calculation means that detects a first in-cylinder pressure that is an in-cylinder pressure at a time that the crankshaft is at an arbitrary crank angle and a second in-cylinder pressure that is an in-cylinder pressure when the crankshaft is at a crank angle reached by rotating by a predetermined angle from the arbitrary crank angle, and calculates a ratio between the first and second in-cylinder pressures; data means that is previously set by expressing a relation between the ratio of in-cylinder pressures and the crank angle in a data format; and characterized in that the control apparatus further comprises: crank angle detection means that detects an angle value of the arbitrary crank angle based on at least the ratio of in-cylinder pressures and the data means, the data means is means that, utilizing a fact that a volume ratio parameter (V n κ /V n+1 κ) or (V n+1 κ /V n κ) that is calculated based on an in-cylinder volume V n at the arbitrary crank angle, an in-cylinder volume V n+1 at the crank angle reached by rotating by a predetermined angle from the arbitrary crank angle, and a specific heat ratio κ is equal to the ratio of in-cylinder pressures, expresses a relation between the volume ratio parameter and the crank angle in a data format. an in-cylinder pressure sensor that is provided in at least one cylinder of an internal combustion engine, and that detects an in-cylinder pressure of the cylinder; rotation angle detection means that detects an angle to which a crankshaft of the internal combustion engine is rotated; pressure ratio calculation means that detects a first in-cylinder pressure that is an in-cylinder pressure at a time that the crankshaft is at an arbitrary crank angle and a second in-cylinder pressure that is an in-cylinder pressure when the crankshaft is at a crank angle reached by rotating by a predetermined angle from the arbitrary crank angle, and calculates a ratio between the first and second in-cylinder pressures; data means that is previously set by expressing a relation between the ratio of in-cylinder pressures and the crank angle in a data format; and characterized in that the control apparatus further comprises: crank angle detection means that detects an angle value of the arbitrary crank angle based on at least the ratio of in-cylinder pressures and the data means, the data means is means that, utilizing a fact that a volume ratio parameter (V n κ /V n+1 κ) or (V n+1 κ /V n κ) that is calculated based on an in-cylinder volume V n at the arbitrary crank angle, an in-cylinder volume V n+1 at the crank angle reached by rotating by a predetermined angle from the arbitrary crank angle, and a specific heat ratio κ is equal to the ratio of in-cylinder pressures, expresses a relation between the volume ratio parameter and the crank angle in a data format. crank angle detection means that detects an angle value of the arbitrary crank angle based on at least the ratio of in-cylinder pressures and the data means, the data means is means that, utilizing a fact that a volume ratio parameter (V n κ /V n+1 κ) or (V n+1 κ /V n κ) that is calculated based on an in-cylinder volume V n at the arbitrary crank angle, an in-cylinder volume V n+1 at the crank angle reached by rotating by a predetermined angle from the arbitrary crank angle, and a specific heat ratio κ is equal to the ratio of in-cylinder pressures, expresses a relation between the volume ratio parameter and the crank angle in a data format.